Utilization of telluride quaternary nonlinear optic materials

ABSTRACT

A solid state laser device made from a nonlinear optic quaternary alloy of Silver, Gallium, Selenium and Tellurium semiconductor material or Silver, Gallium, Sulfur and Tellurium semiconductor material. The Tellurium component in each alloy provides quaternary alloying anion modification of an underlying ternary semiconductor crystal and achieves tuning of the birefringence and tuning of the wavelength passband of the semiconductor material. The tuned quaternary alloy enables beam walkoff-free noncritical phase match operation of the laser device including use of a phase match angle supporting optimum use of the material&#39;s nonlinear properties, maximized useful length of the material crystal, room temperature wavelength changing operation, significantly increased second order nonlinear susceptibility, a factor of ten reduction in the walk-off angle and photon energy conversion efficiencies several times those usually achieved. The Tellurium alloy component also accomplishes shifting of the semiconductor material energy absorption characteristic to avoid a preferred laser pump wavelength energy absorption peak and assists in circumvention of the thermal lensing phenomenon in the crystal. The accomplished laser device provides infrared energy output while operating in for example either the second harmonic generation or the optical parametric oscillation configurations. Examples involving both related materials and the sought after quaternary materials are included.

CROSS REFERENCE TO RELATED PATENT DOCUMENT

The present document is somewhat related to the copending and commonlyassigned patent document “Telluride Quaternary Nonlinear OpticMaterials”, AFD 00334, Ser. No. 09/360,824. The contents of thissomewhat related application are hereby incorporated by referenceherein.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

BACKGROUND OF THE INVENTION

It has been estimated that over sixty percent of the combat aircraftlosses occurring since the 1960's can be attributed to use of infraredresponsive surface to air and air to air missiles. Moreover theexistence of newer more sophisticated generations of these missilesincluding the usually hostile SAM 16, SAM 17 and SAM 18 missiles is nowknown in the western world. Missiles of these latter types areunderstood to include countermeasures capabilities making thetraditional hot flare and similar basic defensive measures against heatseeking missile attack of limited or little value. Although improvedaircraft defensive measures based on laser energy sources have been usedto some degree with respect to such missile weapons, until recentlylaser based infrared countermeasures have been laser source-limited,that is limited in both the available output power level and thespectral coverage achievable. In a very real sense therefore the missileand missile countermeasures battle scene has recently been biased infavor of the missile and its seeker by these power level and spectrumlimitations.

As late as 1997 for example the best available solid state laserinfrared source for missile countermeasures use operated in the range offive watts of average power level and provided little energy output inportions of the infrared spectrum known to be considered in the sensorof later design missiles. Although laser materials based on a certainclass of chalcopyrite alloys have recently made it possible to exceedthis 1997 power level by a factor of four and to achieve peak powers inthe range of a hundred million watts per square centimeter in anonlinear optical crystal material, even higher power levels andoperation in yet inaccessible portions of the infrared spectrum areviewed as desirable improvements in the missile defense art. The presentinvention addresses this area of need and provides an infraredcapability that is useful in areas other than the missile defense field.

Other needs for the present invention are also believed to exist withinthe military art. Following the decrease in tensions between major worldpowers in the 1990's, the threat of chemical or biological weapons usedby smaller potential adversaries has emerged as a remaining and ongoingconcern for the United States and other free world military forces. Withregard to such chemical or biological weaponry it is known for examplethat one chemical warfare agent now available to most potentialadversaries, i.e., the mustard gas of World War I infamy, provides areadily detectable and remotely sensible signature in a specific regionof the infrared portion of the electromagnetic spectrum. This signatureis, however, somewhat limited in bandwidth and therefore requires accessto parts of the infrared spectrum which are not conveniently availablewith many laser sources. Similar limited spectrum signatures arebelieved to exist for other chemical and biological warfare agents. Theremote, safe distance, sensing of such agents is of clear desirabilityin protecting the people and equipment necessary to a militaryoperation. However the variety of threats posed by potential chemicaland biological weapons now suggests that access to virtually unlimitedareas of the infrared spectrum is desirable in the development ofchemical and biological warfare defensive apparatus.

From a third perspective, an equal or perhaps even greater militaryinterest in the infrared spectrum is prompted by the presence of windowsof reduced atmospheric absorption located in certain specific bands ofthe infrared spectrum, especially for example in the 2-6 micrometerwavelength band and in the 8-12 micrometer wavelength band. Thesewindows are believed to offer opportunity for communication,surveillance, and other military and civilian uses not currentlyconsidered feasible. The current situation in infrared spectrumapplications may in fact be comparable with the somewhat recent adventof increased limited spectrum coverage and spectrum agility in the radarutilized microwave frequency parts of the electromagnetic spectrum, adevelopment which has for example made spectral distinction betweenrain, snow and sleet possible in a weather radar system. In addition tomilitary uses there of course exists numerous communication, detectionand object-illumination applications in the non military world which canbe benefited by efficient access to specific and possibly newlyavailable portions of the infrared spectrum.

As a practical matter however infrared emitters usable in the mostdesirable infrared emission source, i.e., usable in the solid statestimulated emission coherent output devices; such as the semiconductorlaser, generate outputs at certain specific wavelengths. Thesewavelengths are, moreover, separated by infrared and other spectralregions in which no desirable efficient direct emission source isavailable. The gas-based carbon dioxide laser is a non-solid stateexample of this situation in that such lasers are for example known tohave strong emission lines residing at wavelengths of 9.3 and 10.6microns. Emissions at wavelengths falling between these two wavelengthsor at specific wavelengths above and below these wavelengths issignificantly less.

The use of wavelength changing devices, devices based on the nonlinearoptic characteristics of certain single crystal semiconductor materials,offers one approach for providing energy at otherwise inaccessiblespectral locations. Prior to the early 1970's there was in fact littleaccess to the wavelengths greater than 4 microns with the availableruby, Neodidium, YAG, Lithium, Argon and other laser materials of commonusage—even with the use of the then available nonlinear and wavelengthchanging materials. In a similar manner, outside the infrared range anabsence of sources in the 1 to 2.5 micron range of wavelength,especially for applications needing tunability, was difficult even whenusing wavelength mixing arrangements. The utility of a wavelengthhalving apparatus may be appreciated by, for example, considering thathalving the wavelength (doubling the frequency) of the 10.6 micronemission line from a carbon dioxide laser provides an output at thewavelength of 5.3 microns, a wavelength at the extreme end of the two tosix micrometer window where the most advanced missiles operate, awavelength which is inaccessible to most laser materials.

The expression “nonlinear optic characteristics” when used in connectionwith the materials of such wavelength changing devices is generallyunderstood to relate to the properties of crystal materials in whichlight transmission characteristics are intensity-dependent, i.e.,materials in which the optical refractive index, n, is a function of theelectric field strength vector, E, of the light wave. Thisrepresentation is of course based on a Maxwell's equation model of lightand the understanding that light energy is fairly described in terms ofelectric field strengths. The light wave index of refraction, n(E), ismoreover represented as the sum of terms in an infinite series expansionof electric field strength vectors taken to the powers or exponents ofzero, one, two and so on with each series term also including a factorof the form n₀, n₁, n₂ and so on representing a refractive index. Inmathematical symbols this relationship may be expressed as:

n(E)=n₀+n₁E+n₂E²+n₃E+  (1)

or alternately as:

n(E)=n₀+Δn  (1a)

Δn=2π/n_(o)[χ⁽²⁾E+χ⁽³⁾E²+χ⁽⁴⁾E^(3 . . .) ]  (1b)

The material property of interest is χ⁽²⁾.

The zero exponent E term, i.e. the n₀ term in the equation 1 series,corresponds to the refractive index used in traditional linear optics,the optics considered in entry level physics courses. The nonlinearmaterials of interest in the present invention are identified as chi twoor second order nonlinear materials, an identification also based onthis infinite series representation of the light wave n(E) andrecognizing that these present invention materials are adequatelycharacterized by a series of the equation 1 type which terminates withthe third term, i.e. terminates with the second power of E, or E squaredterm.

The alloy Silver Gallium Selenide, AgGaSe₂, in single crystal embodimentis presently considered the state-of-the-art carbon dioxide laserfrequency doubling crystal, the preferred crystal for use in laserwavelength change devices such as an optical parametric oscillator, asecond harmonic generator or a difference frequency generator (i.e., anOPO, a SHG or a DFG device; herein devices each referred-to simply as a“laser device”). For present purposes it may be considered that anoptical parametric oscillator provides wavelength doubling or increasingaction, the second harmonic generator provides a wavelength dividing ordecreasing action and the difference frequency generator a sum anddifference frequency mixture output. The term “laser device” is notherein limited to these specific wavelength changing arrangementshowever and may also identity other stimulated energy, coherent outputapparatus. In other words the present invention is deemed not to belimited to a optical parametric oscillator, a second harmonic generatoror a difference frequency generator.

As may be noted in the preceding and several other earlier paragraphsherein, the once universal convention of capitalizing the names ofperiodic table elements is observed in the present document.Additionally, the Silver Gallium Selenide, AgGaSe₂, material is forexample recognized as being formally classed as a “di-selenide”material. In the interests of brevity and simplicity however such formalreference is omitted herein and this material as well as the othersimilarly classifiable materials are herein referred-to by the shorterSilver Gallium Selenide and similar names.

The photon conversion efficiency of this AgGaSe₂ state-of-the-art andmost widely used infrared nonlinear optical crystal material is limitedin wavelength doubling service because of its non optimal birefringencecharacteristic. In view of such birefringence limitation, laserapparatus use of this material results in a crystal phase matching anglefailing to effectively utilize the available optical nonlinearity of thematerial, a phase matching angle also allowing excessive walk-off of thesignal and pump beams within a AgGaSe₂ crystal and the accompanyingsevere loss of photon conversion efficiency. The terms “birefringence”and “walk-off” are believed known in the art and are discussed anddefined in some detail in the ensuing paragraphs of this disclosure.Relatively low thermal conductivity and the resulting thermal lensingtendency is another area of difficulty with this AgGaSe₂state-of-the-art nonlinear optic material. Yet another limitation ofAgGaSe₂ is excessive photon energy absorption at a wavelength of twomicrons, a limitation which limits its performance in two micron-pumpedoptical parametric oscillation-based laser systems.

Other nonlinear optical materials are of course available for possibleuse in overcoming these difficulties with Silver Gallium Selenide. Somesuch materials together with Silver Gallium Selenide are classified aschalcopyrite materials in a broad sense of the term chalcopyrite.Included in these other materials are for example Silver GalliumSulfide, AgGaS₂; Zinc Germanium Phosphide, ZnGeP₂ and Cadmium GermaniumArsenide, CdGeAs₂. With the possible exception of the first of thesematerials, known limitations of the material make these other materialseven less desirable in practice for present need laser wavelengthchanging use and have therefore contributed to the AgGaSe₂ materialhaving its current state-of-the-art status. The Silver Gallium Sulfide,AgaS₂, material, when modified into a somewhat related four element orquaternary alloy as disclosed herein, is deemed a viable andcomplementary, material for use in nonlinear optical apparatus,especially in view of the transparency in the red end of the visiblewavelength portions of the optical spectrum it provides and theresulting wavelength-change coverage of an additional spectral region.

The Silver Gallium Selenide material is considered in significant detailin the first of the examples included in the present patent document. Asrelated subsequently herein this detailed consideration of SilverGallium Selenide is partly based on it being a nonlinear chalcopyritematerial of close relationship to one of the quaternary alloys ofprinciple focus in the present patent document—and therefore of interestin the present “closely related material” disclosure of this quaternaryalloy. The consideration of Silver Gallium Selenide herein is also basedon the fact that the properties of this three element or ternary alloyare in some specific characteristics similar to those of one focusedupon quaternary material, ie., AgGa(Se_((1−x))Te_(x))₂. Moreover thepresent document interest in the Silver Gallium Selenide ternarymaterial is also based on the fact that it is a viable startingcomponent for fabricating this one of the focused upon quaternarymaterials. Similar relationships are seized upon in the present patentdocument with respect to another focused upon quaternary material,Silver Gallium Sulfide, AgGa(Se_((1−x))Te_(x))₂, as is described indetail in the following paragraphs and the examples disclosed below.

Returning to the present background of the invention discussion, in viewof little more than the recited limitations of what is considered to bethe state of the art best infrared wavelength changing material, thereis clearly need in the laser apparatus art for a frequency doublingmaterial offering a more desirable combination of performancecharacteristics than has heretofore been available. The presentinvention is believed to provide desirable answers for this need in theform of Tellurium-inclusive quaternary alloy chalcopyrite materials and.their utilizations. The present invention focuses on twoTellurium-inclusive alloys including the quaternary alloys SilverGallium Selenide Telluride, AgGa(Se_((1−x))Te_(x))₂ and Silver GalliumSulfide Telluride (i.e., Silver Thiogallate Telluride),AgGa(Se_((1−x))Te_(x))₂. These quaternary alloys, are consideredrelevant over a range of Selenium/Tellurium and Sulfur/Telluriumcompositions as is indicated by the complementary x subscript notationsin these chemical formulas. The present invention is however deemed notto be limited to these specific Tellurium alloys.

SUMMARY OF THE INVENTION

In the present invention the periodic table element Tellurium has beenadded to chalcopyrite single crystal nonlinear optical materials inorder to provide the described new and superior performing infraredlaser devices. The chalcopyrite material may be a quaternary alloy ofeither Silver Gallium Sulfur and Tellurium or Silver Gallium Seleniumand Telluriumn These materials, although somewhat similar, providewavelength changing accommodation of differing infrared wavelengthregions. By adding the periodic table element Tellurium, thebirefringence of the resulting single crystal alloy can be tuned to adesirable value for a given wavelength multiplying or dividing action inthe disclosed laser devices. This Tellurium addition also contributes toparallel input and output beam travel in the single crystal material, atravel maintainable without beam walkoff degradation. The addition ofTellurium also shifts an absorption characteristic of these materials tolonger wavelengths and thereby limits energy losses and assists incontrolling beam quality degradation caused by thermal lensing in thelaser device material.

It is an object of the present invention therefore to effectively use aTelluride-inclusive quaternary alloy nonlinear optical materials oftunable birefringence and enhanced solid state infrared laser efficiencycapability in an improved laser device.

It is another object of the present invention to provide advantageousutilization of a single crystal nonlinear optical material employing theelement Tellurium in a quaternary alloy single crystal structure.

It is another object of the invention to provide laser devices enhancedby extension of the chalcopyrite family of nonlinear optical materialsto include Tellurium-containing quaternaxy materials.

It is another object of the invention to provide laser wavelengthconversion apparatus of significantly enhanced photon energy conversionefficiency.

It is another object of the invention to provide laser apparatusimproved by optic material of decreased photon energy absorptioncharacteristic in certain desired infrared wavelength regions.

It is another object of the invention to provide a laser apparatusincorporating optical material of selectable birefringence capability.

It is another object of the invention to provide enhanced wavelengthconversion efficiency through laser device utilization of increaseduseful length in a wavelength conversion crystal.

It is another object of the invention to provide a laser device based onnonlinear optical material having traditional cation alloying replacedby lower concentration anion alloying.

It is another object of the invention to use an indium free quaternarynonlinear chalcopyrite crystallographic optical material in a laserdevice.

It is another object of the invention to provide a laser device using anonlinear optical material achieving non critical phase match photonenergy conversion.

It is another object of the invention to provide a laser device using anonlinear optical material supportive of non critical phase matching andidentical beam trajectory operating conditions.

It is another object of the invention to provide a solid state laserdevice with nonlinear optical material enabling photon energy conversionefficiency levels in the range of two to three times those ofconventional solid state laser devices.

It is another object of the invention to provide a laser apparatus usinga nonlinear optical material capable of achieving improved input andoutput wave phase matching with resulting energy conversion efficienciesapproaching ninety percent.

It is another object of the invention to provide a coherent energywavelength conversion apparatus having nonlinear optical materialcapable of generating beam walk-off-immune energy conversion.

It is another object of the invention to provide a laser apparatus withnonlinear optical material capable of both second harmonic generationand optical parametric oscillation operating modes.

It is another object of the invention to provide a nonlinearoptical-based material high power wavelength-agile laser.

It is another object of the invention to provide characteristicinformation from which the properties of other nonlinear infrared laserdevices can be tailored to specific operating characteristic regions.

It is another object of the invention to provide a laser device usingthe nonlinear optical material Silver Gallium Selenide TellurideAgGa(Se_((1−x))Te_(x))₂.

It is another object of the invention to provide a laser device usingthe nonlinear optical material Silver Gallium Sulfide Telluride,AgGa(Se_((1−x))Te_(x))₂.

Additional objects and features of the invention will be understood fromthe following description and claims and the accompanying drawings.

These and other objects of the invention are achieved by laser apparatuscomprising the combination of:

a source of coherent radiation pump energy of first output wavelengthcharacteristic;

a nonlinear optics wavelength changing semiconductor crystal elementdisposed intermediate said source of coherent radiation pump energy anda infrared wavelength optical output port of said laser apparatus;

said nonlinear optics wavelength changing element including aTellurium-comprised single crystal quaternary alloy chalcopyritesemiconductor material having a crystal structure comprising:

a crystalline cubic lattice (304) located at an intersection (306) of100, 001 and 010 coordinate axes (302, 303, 305) said lattice (304)having lattice initial plane faces (308, 310, 312) received in planesdefined by each 100-001, 100-010 and 010-001 axis pairs, havingsublattice-defining lattice mid planes (314, 316, 318) distal to andparallel with lattice initial planes (308, 310, 312) respectively andhaving exterior face planes (320, 322, 324) distal to and parallel witheach lattice initial plane (308, 310, 312) and each lattice mid plane(314, 316, 318) when viewed along any of three paths (326, 328, 330)parallel to a 100, 001 and 010 acres, paths orthogonal to 001-010,100-010, and 100-001 planes respectively;

Gallium atoms (332) located in each sublattice corner of a lattice midplane lying along said 100 axis (302) parallel with said 001-010 plane;plus

Gallium atoms (334) located at each sublattice center in the latticeinitial and exterior planes (312, 324) lying along said 100 axis (302)commencing at said 001-010 plane; plus

Gallium atoms (336) located at sublattice centers of said initial, saidmid and said exterior planes (310, 316, 322) within a first half (337)of said cube lattice (304), a half located parallel to and adjacent said001-010 plane, along said 100 axis (302); plus

Silver atoms (338) located at each sublattice corner in lattice initialand exterior plane faces (320, 324) disposed along said 100 axis (302)parallel to said 001-010 plane; plus Silver atoms (340) located atsublattice centers of said initial, said mid and said exterior planes(310, 316, 322) along said 001 axis within a second half (342) of saidcube lattice, a half located parallel to and distal of said 001-010plane along said 100 axis (302); plus

one of differential number-quantity, similarly located, Sulfur andTellurium atoms (343) and differential number-quantity, similarlylocated, Selenium and Tellurium atoms (343) said differentialnumber-quantity atoms (343) being received in planes intermediate saidlattice initial plane, said lattice mid plane and said lattice externalplane in mediate planes lying along each of said 100, 001 and 010 axes,said differential number-quantity atoms (343) being disposed in orderedarray in random fill anion lattice patterns paralleling each of said100-010, 100-001, and 010-001 planes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a wavelength changing laser device inwhich nonlinear optical materials may be used.

FIG. 2 shows a family of design nomogram curves relating inputwavelength, phase matching angle and Tellurium content for onequaternary chalcopyrite material according to the present invention.

FIG. 3a shows a threedumensional perspective view of a chalcopyritecrystal.

FIG. 3b shows an elevation view of a chalcopyrite crystal.

FIG. 3c shows a rotated three dimensional elevation view of achalcopyrite crystal.

FIG. 4 shows an aircraft countermeasures scene in which photon signalsoriginating in a wavelength changing nonlinear crystal according to thepresent invention are used.

FIG. 5 shows a battlefield surveillance scene in which photon signalsoriginating in a nonlinear wavelength changing crystal are used.

FIG. 6 shows a representation of the indices of refraction for ordinaryand extraordinary waves in a negatively birefringent nonlinear uniaxialcrystal such as the materials disclosed herein.

FIG. 7 shows the relationship between birefringence and wavelength forfour chalcopyrite materials relating the quaternary alloys of thepresent invention.

FIG. 8 shows experimental n_(e) and n_(o) refraction indices forAgGaTe₂, a material relevant to the present invention quaternarychalcopyrite alloys.

FIG. 9 shows room temperature optical transmission data for two samplesof the FIG. 8 AgGaTe₂ materin under differing conditions.

FIG. 10a shows one improvement in nonlinear crystal performanceachievable with quaternary chalcopyrite alloys.

FIG. 10b shows another improvement in nonlinear crystal performanceachievable with quaternary chalcopyrite alloys.

FIG. 11 shows the relationship between birefringence, wavelength andtemperature for the chalcopyrite material AgGaTe₂, a material usableboth in a wavelength changing element and as a component material forother birefringent chalcopyrite materials considered herein.

DETAILED DESCRIPTION

The present invention is primarily concerned with laser apparatus usingthe quaternary chalcopyrite alloys Silver Gallium Selenide Telluride,AgGa(Se_((1−x))Te_(x))₂ and Silver Gallium Sulfide Telluride,AgGa(Se_((1−x))Te_(x))₂ for wavelength changing. The following detaileddescription relating to this apparatus is based on the best presentlyavailable information relevant to these quaternary materials i.e., basedon characteristics of the plural ternary chalcopyrite materials ofrelated properties and usability as precursors of the quaternary alloys.The chalcopyrite nature of these precursor alloys together with thefrequently accomplished extrapolation of characteristics within closelyrelated material families suggests the viability of this “closelyrelated materials” discussion.

An additional aspect of this disclosure arises from the presentdocuments being the believed first patent disclosure/publication ofdetailed characteristics relating to one of these precursor chalcopyritematerials, the material Silver Gallium Telluride. Although this materialis known in the laser device art, limited knowledge concerning thismaterial and previous difficulty in fabricating samples sufficientlylarge and sufficiently pure for its characteristic measurement haveheretofore made its characteristics unavailable and thereby made thematerial itself of generally limited utility. Moreover, applicants havenoted below a belief that alloys inclusive of the element Tellurium havebeen somewhat passed over in the nonlinear optical art.

Applicants' understanding of the prior art in the laser wavelengthdoubling field therefore suggests the periodic table element Telluriumand especially quaternary chalcopyrite alloys of this element have beenlargely and surprisingly overlooked in the realization of infraredwavelength doubling materials. This overlooked status includes the largerange of specific different quaternary alloys represented by thechemical symbols AgGa(Se_((1−x))Te_(x))₂ and AgGa(Se_((1−x))Te_(x))₂(wherein x is a value between 0.02 and 0.98) and even though the mostoptimum Tellurium content in such alloys appears to fall in the rangearound eighteen (18) percent as is more fully discussed subsequentlyherein. This overlooked status of Tellurium alloys is believed even moresurprising in view of extensive study of the similar alloys, AgGaS₂ andAgGaSe₂, alloys extensively discussed in the technical literature andnow readily available commercially.

Terminology and Underlying Concepts

The exceptional text “Handbook of Nonlinear Optical Crystals” by V. G.Dimitriev, G. G. Gurzadyan and D. N. Nikogosyan published in a firstedition in 1991 by Springer-Verlag of Berlin, Heidelberg, New York etc.is believed helpful in further understanding nonlinear optical concepts,in defining terminology, disclosing characteristics and explainingtheoretical and quantitative considerations relevant to the presentinvention. Chapter two titled “Optics of Nonlinear Crystals” and inparticular section 2.3 of chapter 2 concerning uniaxial crystals arebelieved notably of interest. A second edition of this handbook waspublished in 1997 and includes specific materials data but does notappear to change the chapter 2 information. Numerous references to eachof the nonlinear optic materials AgGaS₂, AgGaSe₂ and Tellurium appearunder the specific chemical symbol names in the index of the 1991 textand an extensive list of reference publications is provided. Notably theherein disclosed quaternary alloys Silver Gallium Selenide Telluride,AgGa(Se_((1−x))Te_(x))₂ and Silver Gallium Sulfide Tellurkle,AgGa(Se_((1−x))Te_(x))₂ appear absent from the handbook texts. TheCopyrighted Handbook of Nonlinear Optical Crystals texts are presentlyreferred-to as “the Springer-Verlag Handbook” and the text of eachhandbook is hereby incorporated by reference herein. In addition to theSpringer-Verlag text numerous other patent and publication referencesare identified in subsequent paragraphs of this document; each of thesereferences is also hereby incorporated by reference herein.

Operation of a nonlinear optical device under improved phase matchingenergy exchange conditions between input and output optical waves is anotable attribute of the present invention. Such improved phase matchingoperation is known in the art to increase energy exchange into an outputwave by several magnitudes over that of a conventional poorly phasematched energy exchange arrangement. In general terms this phase matchedoperation involves causing the input and output waves of the nonlinearcrystal to travel through the crystal at the same speed even though theyare of different frequency or different wavelengths. In other words onecondition for efficiently phase matched operation is for the two wavesin a nonlinear crystal to experience the same index of refractionnotwithstanding their different wavelengths. In the present inventionthis similar index of refraction and similar velocity are achievedthrough the “trick” of employing differing polarizations between inputand output waves of the nonlinear crystal and thereby achieving maximumenergy transfer between the waves. Such operation may be viewed ascausing the input wave to vibrate the bonded electrons of the crystalatoms in such a manner as to enable the output wave to absorb thevibration energy with greatest efficiency.

Another significant aspect of the present invention is concerned withoperating the involved solid state laser device under conditions ofnoncritical phase matched energy transfer between the differingwavelength input and output beams of the laser device. In particularthis noncritical phase matched operation enables the advantages of beamwalkoff immunity, higher photon energy conversion efficiency, convenientoptical processing of idler and primary output beams from the device,output wavelength tuning of a particular laser device, lowered energylosses and reduced thermal lensing difficulties and possibly otheradvantages in the laser device. Achievement of noncritical phase matchedoperation and the resulting performance advantages is made possible byan appropriate selection of laser device materials and materialcomponent fractions together with precise selection of input beampositioning with respect to the laser device crystal in order that theoutput path lie along a major axis of the crystal. Additionalconsiderations regarding this mode of laser device operation are to befound in chapter 2 of the Springer-Verlag Handbook above identified.FIGS. 2.5 and 2.7 of this text and the related discussion of uniaxialcrystals in section 2.3 appear particularly relevant.

Wavelength changing in a laser device optical nonlinear crystal isenabled by the birefringence characteristic of the nonlinear crystalmaterial. Two separate rays propagate through a birefringent crystalwith each such ray being linearly polarized at a right angle to theother. These two rays, the ordinary ray and the extraordinary ray, areinfluenced differently during their travel through nonlinear crystalmaterial. The two rays, for example, may encounter two differentpropagation velocities and two different indices of refraction, i.e.,indices of n_(o) and n_(e) for the ordinary and extraordinary raysrespectively. Additionally the ordinary ray propagates perpendicularlyto its wavefront while the extraordinary ray does not. With respect tothe present invention it is significant that the angular separationbetween the two rays in a crystal depends on the direction in which thelight travels through the crystal relative to the crystal's optical axiswith the exception of light traveling parallel or perpendicular to theoptical axis where either polarization propagates parallel to itswavefront.

In order to achieve usable wavelength changing efficiency in a crystalit is needed for the employed crystal material to be possessed of asufficient degree of this birefringence response to the light energyapplied to the crystal. Although many optical materials exhibit somedegree of birefringence only a selected group of materials providesufficient birefringence to be of interest in practical nonlinearoptical devices. The chalcopyrite family of materials includes asignificant portion of these materials of interest—when the termchalcopyrite is understood to indicate a group of crystal materialshaving lattice structure resembling that of the original or classicchalcopyrite, i.e., resembling the yellow crystal CuFeS₂ material,without, however, limitation to the CuFeS₂ material.

The birefringence characteristic therefore indicates the opticalmaterial is inherently optically anisotropic in nature, i.e., that itsoptical properties are not the same in all directions of a sample.Birefringence may result from either different separation betweenneighboring atoms in a crystal or from the bonds holding neighboringatoms together having different characteristics in different directions.An alternate description of the crystal is that the atoms in such acrystal are more closely located in some planes through the materialthan in other planes and therefore the optical properties of the crystaldiffer in different directions. The birefringence characteristic of analloy material also is responsive to both the alloy composition of thematerial and the temperature of the material. In the present inventionthis alloy composition sensitivity is used to particular advantage.Temperature sensitivity of the birefringent nonlinear material isaccommodated in a laser device by thermal conductivity aspects of thematerial, by selecting the material composition in response to theavailable ambient environment or by temperature control of the materialor some combination of these arrangements as is known in the thermalart. Externally sourced cooling and heating may then be used to maintainthis desired crystal temperature.

Birefringence is often defined quantitatively in terms of the differencebetween two indices of refraction, n_(o) the index of refraction for anordinary wave and n_(e) the index for an extraordinary wave in thematerial. If n_(o) is the larger of the two indices, and the ordinaryleave thereby travels slower than the extraordinary wave in the crystal,as is the case of primary interest for the present quaternarychalcopyrite alloys, then the material is said to have a negativebirefringence characteristic. The indices of refraction for negativebirefringence material is often described in terms of a geometricellipse, representing the n_(e) refractive index, being inscribed withina circle representing the n_(o) refractive index, this representation isdiscussed more fully in the following paragraphs herein and isillustrated in FIG. 6 of the drawings.

Efficient energy transfer between input and output beams of a nonlinearcrystal requires these two beams to remain in a parallel and closelyaligned condition while passing through the crystal, i.e., requiresabsence of the beam walkoff condition. Negative birefringence whereinn_(o) is greater than n_(e) is particularly of interest with respect tothe present invention since it enables laser device operation in the“noncritical phase matched operating mode”, a mode in which angulardivergence walkoff between the ordinary and extraordinary beam outputsof the nonlinear crystal is absent. Both the n_(e) and n_(o) indices fora birefringence material vary with the composition of the material,i.e., with the Tellurium content in the present quaternary alloyinstance. Stating this relationship in a differing manner it can be saidthat achievement of specific n_(e), n_(o) and n_(e), −n_(o) values andrealizing the desired noncritical phase match condition is possiblethrough selection of the crystal material or the alloy composition.Materials in which n_(e) exceeds n_(o) are conversely said to havepositive birefringence; such positive birefringence material does notenjoy the noncritical phase match operation nor walkoff immunitycharacteristics.

The phenomenon of walkoff or Poynting vector walkoff in a nonlinearoptical device is believed to be well understood in the optical art andis therefore recited without substantial elaboration in the presentpatent document. Chapter 2 in the Springer-Verlag text identified hereinincludes several mathematical equations used to quantitatively evaluatethe walkoff phenomenon in specific settings. The phenomenon is alsoconsidered at length in several issued U.S. Patents including the U.S.Pat. Nos. 5,847,861 and 5,365,366 of J. D. Kafka et al., U.S. Pat. No.5,732,095 of P. Zorabedian, and U.S. Pat. No. 5,297,156 of D. A. G.Deacon.

The differing velocity of a light wave in different directions in ananisotropic crystal is therefore significant with respect to the presentinvention. These differing velocities are often described by way ofconsidering the differing indices of refraction for a linearly polarizedlight vector in the three coordinate directions of the anisotropicmaterial as three defming axes of an “index ellipsoid”. Planarellipsoidal sections taken through the origin of the “index ellipsoid”then defme the index of refraction for a particular direction throughthe material. This ellipsoid-based, essentially geometric, analysisgives rise to the frequent appearance of trigonometric and second powerterms in the mathematical expressions characterizing nonlinear opticalmaterial properties. Additional information regarding these matters isto be found in chapter 2 of the above identified Springer-Verlaghandbook text.

Such ellipsoidal sections are each themselves of elliptical shape as isrepresented in FIG. 6 herein and the major and minor axes of suchellipse represent the refractive indices or fast and slow wavevelocities relevant to a given direction through the material, i.e.,refractive indices or fast and slow wave velocities for light beamedalong a normal to the plane of the major and minor ellipse axes. Whenthe indices of refraction for a certain type of crystal are not unequalin these three coordinate directions, the index ellipsoid ischaracterized by an axis of revolution or axis of symmetry and has theshape of an ellipsoid of revolution; light is propagated along this axisof revolution as if the material is not anisotropic but is isotropici.e., its velocity is independent of polarization state and the twoindices of refraction are equal. The ellipsoid section taken at rightangle to this axis of revolution is of circular shape. Crystals of thismaterial, e.g., crystals of the materials used in the present invention,are said to be of a uniaxial nature. In contrast with this uniaxialmaterial, the index ellipsoid in nonlinear optic crystals of lowsymmetry, i.e., biaxial crystals, includes three unequal axes. Thematerials of interest in the present invention are of this uniaxialtype.

Refractometry methods kown in the art may be used in determining theindex of refraction of the materials discussed herein. Suchrefractometry must, of course, consider the birefringencecharacteristics present and recognize that refractometry techniquesdependent on simple Snell's law refraction relationships are not validfor the extraordinary beam output of a chalcopyrite or other nonlinearmaterial sample. The present inventors have experienced satisfactoryrefractometry measurement results using prism spectrometer measurementsof the minimum deviation angle D, provided by a chalcopyrite materialsample and the fundamental relationship:

D=θ₁+θ₂+A  (2)

where θ₁ and θ₂ are the angles between incident and exit measurementbeams and the normal to the prism surface at the point of entrance orexit and A is the prism angle between entrance and exit surfaces. Thisequation 2 relationship may be supplemented with additionaltrigonometric relationships including:

n=[sin ½(A+D)]/sin ½A  (3)

so long as the measurement procedure is limited to the ordinary rayoutput of the prism sample where Snell's law is relevant. The symbol nis the sought-after index of refraction in equation 3. Otherrefractometry measurements based on for example real and apparentthickness dimensions of a planar sample and the critical angle of totalreflection are known in the art.

In the interest of completeness several additional equations consideredhelpful in defming the characteristics of both positive and negativebirefringence materials are presented here. Additional equationsrelevant to specific materials and their characteristics are alsoincluded in the later examples of this document. The first of theseadditional equations concerns the effective or usable nonlinearcoefficient of a chalcopyrite material with a positive birefringence, amaterial such as the later example discussed AgGaTe₂ and AgInSe₂materials. For the usually assumed symmetry conditions (i.e., d₃₆=d₁₄)and for Type I phase matching this nonlinear coefficient is expressedby:

(d_(oee))_(eff) or (d_(eeo))_(eff)=d₃₆ sin2θcos2φ=d₃₆ sin2θ(usual φchoice)  (4)

where θ is the internal angle between the optical or c-axis and thedirection of propagation for which phase matching occurs, φ is theazimuthal angle which may be chosen such that cos2φ is one, andd₃₆=(χ₁₂)/2 is the nonlinear optical coefficient χ⁽²⁾ is the secondorder nonlinear susceptibility. The nonlinear coefficient isadditionally discussed in the reference by J. L. Shay and J. H. Wernick,Ternary Chalcopyrite Serniconductors: Growth, Electronic Properties, andApplications (Pergamon, New York, 1975).

The corresponding figure of merit for energy conversion efficiency (FOM)is given nominally by,

FOM=(d₃₆sin2θ)²/n³,  (5)

where n is the average index of refraction well below the band gap. Notethat the FOM is largest for θ=45° and zero for θ=90° and it scales as(d₃₆)².

The phase matching angle is uniquely determined by the refractiveindices and their dispersion which are themselves almost totallydetermined by the fundamental band structure. The angle is given by thefollowing expression for n_(e) >n_(o), the condition known as positivebirefringence, and for Type I phase matching which makes maximum use ofthe available birefringence.

sin²θ=(n_(e)/n_(o)′)²[(n_(o)′+n_(o))(n_(o)′−n_(o))/(n_(e)+n_(o))(n_(e)−n_(o))]or  (6)

sin²θ≅(n_(o)′−n_(o))/Δn  (7)

The effective or usable nonlinear coefficient of a chalcopyrite with anegative birefringence such as the later example disclosed AgGaSe₂ orAgGaS₂ or the AgGa(Se_((1−x))Te_(x))₂ or AgGa(Se_((1−x))Te_(x))₂materials, the FOM and the birefringence angle each for the usuallyassumed symmetry conditions (i.e., d₃₆=d₁₄), and for Type I phasematching is:

(d_(oee))_(eff) or (d_(eeo))_(eff)=d₃₆ sinθ sin2φ  (8)

FOM=(d₃₆ sinθ)²/n³  (9)

sin²θ=(n_(e)′/n_(o))²[(n_(o)′+n_(o))(n_(o)′−n_(o))/(n_(e)′+n_(o)′)(n_(o)′−n_(e)′)]or  (10)

sin²θ≅(n_(o)′−n_(o))/(−b), b=(n_(e)′−n_(o)′)  (11)

Utilizations

FIG. 1 in the drawings shows a block diagram of a laser apparatus inwhich chalcopyrite materials of the herein disclosed type may be used ina wavelength changing arrangement to achieve infrared spectrum outputenergy from a convenient coherent energy source. In the FIG. 1 drawing alaser source 104 is used to provide pump energy to a nonlinearwavelength changing crystal 110 of the presently discussed type. Thecrystal 110 is shown in FIG. 1 to be disposed between two mirrors 106and 108, mirrors having the partial reflection and partial transmissioncharacteristics normally used with laser cavities. The c-axis or opticalaxis of the chalcopyrite material crystal 110 is shown at 111 in FIG. 1together with an angle θ relationship at 123 between the axis 111 andthe selected optical path 121-122 through the crystal. As indicated bythe differently configured dotted lines at 112, 114, 116 and 118 in theFIG. 1 drawing, output energy from the crystal 110 may be one of severaldiffering wavelengths as is determined by the wavelength changingconditions established in the cavity containing the crystal 110. In FIG.1 these different output wavelengths are segregated in physical locationby the conventional spectrum distributing action of the prism 109.Generally such different unitary outputs are the result of operating thecrystal 110 and the cavity it comprises with slightly differing valuesof the angle θ. Alternately it may be stated that the FIG. 1 operationinvolves rotating the crystal 110 to get the proper angle θ to providethe desired output wavelength λ.

The laser 104 in the FIG. 1 apparatus is indicated to be of the carbondioxide type, a device well known in the laser art and a laser havingoutput energy located at the indicated 10.6 micron and 9.3 micronspectral lines. Each of these spectral lines, for example, leads to adesirable doubled infrared wavelength in the FIG. 1 apparatus outputenergy. By way of the above discussed careful selection of crystalangles and with selection of mirror reflectivity, either of these carbondioxide laser spectral lines or other spectral components of the laser104 output may be selected for wavelength doubling (or for another ofthe above recited OPO, SHG and DFG device three types of spectrallocation changing mechanisms) within the nonlinear crystal 110 and itssurrounding cavity. According to one aspect of the present invention,the photon conversion efficiency between the energy of the input beam120 and that of an output beam at 112, 114, 116 and 118 is significantlylarger than has been achievable with the wavelength conversion materialsavailable heretofore. The FIG. 1 wavelength changing apparatus may beembodied into any of several end use devices, two such end use devicesof present inventor interest are disclosed in connection with thedrawings of FIG. 4 and FIG. 5 herein.

Even though noncritical phase matched operation of a laser device, as isespoused in the present discussion, significantly stabilizes operationof a FIG. 1 laser device with respect to beam walkoff tendencies suchstabilization has practical limits and the maintenance of a selectedoperating temperature in the crystal 110 can be a significantconsideration in a FIG. 1 apparatus. Indeed many such laser devices usetemperature change of the crystal 110 as a mechanism for tuning crystalcharacteristics such as birefringence or refractive indices to aselected range of values. Temperature changes in the crystal 110 mayalso result in the phenomenon of thermal lensing wherein crystal shapedistortions result in lensing characteristics, characteristics that areusually a problem in a use apparatus. Good thermal conductivity withinthe crystal material is desired in order to conduct away heat generatedby energy loss mechanisms in the crystal during both high instantaneouspower peaks and during high duty cycle usage.

One general procedure for accommodating these thermal characteristics ofa nonlinear optical device crystal is enhanced by the variety of crystalcharacteristics made available with crystal 110 Tellurium content changeas is shown in the nomograph of FIG. 2 for example. According to thisarrangement curves of the FIG. 2 type (curves relevant to thetemperature of the environment contemplated for a particular FIG. 1apparatus in lieu of the FIG. 2 room temperature curves) may be used totailor a particular crystal used at 110 in FIG. 1 to its normalenvironment. Such arrangement might for example provide a set of crystalcharacteristics for a low, near absolute zero degrees, temperature for alaser device operated in a shaded outer space location, or a set of −30degree Fahrenheit characteristics for a missile operated exclusively athigh altitude or a set of characteristics tailored for the temperatureof boiling water where atmospheric pressure water phase change is usableas a crystal coolant. In the FIG. 1 drawing the box 124 is employed torepresent heating or cooling crystal temperature maintenancearrangements possibly needed during uses of the FIG. 1 apparatusaccording to these temperature considerations.

One military use of the FIG. 1 wavelength shifting laser device and itspresently disclosed component materials is represented in the FIG. 4drawing. In this drawing a tactical aircraft 400 is shown in a defensiveencounter wherein it has been identified as a target by two heat seekingguided missiles 402 and 408. The missiles 402 and 408 could be of eitherground-based or airborne origin and are presumed to have been designatedas a threat by electronic and human-controlled systems included withinthe aircraft 400. In an attempt to defeat the locked-on status of themissiles 402 and 408 with respect to the aircraft 400, the aircraft haslaunched an active defensive device 404, a device emitting infraredcountermeasures signals received by the heat seeking guidance system ofthe missiles 402 and 408. These infrared countermeasures signals may beprovided with a plurality of signature characteristics intended to forexample deceive the missile 402 and lead it away from the aircraft 400or cause premature/distal and harmless detonation of the missile'swarhead.

Since the defensive device 404 may include a propulsion system of itsown and thereby appear as a viable heat source target to theantiaircraft missile 402, some embodiments of the missile 402 can beneutralized as a threat to the aircraft 400 by nothing more than thedefensive device 404 providing its own reasonable heat signature. Laterdeveloped and more sophisticated antiaircraft missiles, however, arebelieved capable of distinguishing the heat signature of the aircraft400 from that of the defensive device 404 (these later antiaircraftmissiles are also of course capable of ignoring the heat signature ofthe simplified burning flare defensive devices used in world war II andthereafter). Part of this more sophisticated signature distinguishingcapability is attributable to improved partitioning of the infraredspectrum within the heat sensing transducers of the antiaircraft missile402, in other words to a more precise spectral bandwidth selectivity anduse of specific “unusual” spectral wavelengths in the heat sensor of themissile 402. In the FIG. 4 scene the additional missile originallythreatening the aircraft 400, the missile 408, is shown in a neutralizedposition and orientation it may occupy following some earlier receipt ofguidance system deceiving infrared signals from the defensive device404. Such signals may for example have been received by the missile 408in the region 410 and resulted in a disturbance of the locked on targetcondition achieved by the missile 408 guidance system. The missile 402will prospectively encounter a similar fate in due course.

One use of the present invention wavelength changing chalcopyritecrystal materials is therefore seen to reside in the capability of thesematerials to provide usable access to new portions of the infraredspectrum for defensive devices of the type shown at 404 in FIG. 4.Specifically, with use of the present invention materials it becomespossible to tailor one or more electrically generated infrared signalsof the type represented at 406 in FIG. 4, i.e., electrically sourcedwavelength-shifted laser output signals emitted by the defensive device404, to essentially any desired portion of the infrared spectrum. Withthe capabilities disclosed herein it is, of course, also possible toprovide multiple spectrum signatures, time varying signatures andotherwise enhanced signals at 406 for the defensive device 404. Indeedthe present invention materials provide efficient usable access to anypart of the infrared spectrum that may be utilized by newly emergingheat seeking missile transducers.

FIG. 5 in the drawings shows another possible military use of the FIG. 1wavelength shifting laser device and its component materials. In theFIG. 5 drawing a ground base military scene involving possible biohazard elements such as toxic gasses or biological agents isrepresented. In the FIG. 5 scene both the personnel within the battletanks 500 and 502 and other nearby persons, such as the military scout504 viewing from some safe remote location, have need to know if thedispersed material within the cloud 506 is of natural and safe origin oris the result of enemy activity. Since the battle tanks 500 and 502 canbe provided with filters and air sources otherwise isolated from theirenvironment the more pressing need represented in the FIG. 5 scene canperhaps be attributed to the scout 504 and his companion ground forcepersonnel. Indeed such remote safe distance identification of biologicalhazards is seen as an especially significant use of the presentinvention laser devices and materials—uses inclusive of both militaryand non military applications of the disclosed materials, methods andapparatus.

A significant likelihood for usage of the present invention insituations such as those represented in FIG. 5 is to be found in thefact that certain biological hazard agents such as the mustard gas ofearlier warfare common usage (as well as present day terrorist interest)is now so easily fabricated in “fertilizer” and “pharmaceutical”infrastructure settings as to pose a low cost and easily availablethreat to many people in the world. Countermeasures for use against suchagents are, however, known in the art and can be quite effective in thepresence of early detection of the hazard. This is again a realm forpossible use of the present invention materials methods and apparatus.

With regard to this mustard gas agent it is known in the art, forexample, that this material has a readily discernible signature locatedin the infrared spectrum regions made accessible through use of thewavelength changing capability of the present invention. Such infraredsignatures are believed to exist for other and indeed many or most ofthe known biohazard agents including those of bacterial and virusorigin. The flexibility of tailoring an illumination device usable by aperson such as the FIG. 5 scout 504 to a specific infrared wavelengthcharacteristic of each such agent or to a plurality of wavelengthscharacteristic of several such agents is therefore of significantmilitary utility. Once such infrared signatures are identified, anextended line of sight path such as the path 508 in FIG. 5 may be usedto remotely identify the presence of such agents from a safe distance.

Notably such FIG. 5-represented identification can be based on materialabsorption of specific wavelength energy sourced from the presentinvention materials or on reflection of this energy back to a sendinginstrument or on some combination of wavelength dependent reflection andabsorption. Parenthetically it may be of interest to note that althoughsuch FIG. 5-related optical detection of gases and aerosols is madepossible or significantly enhanced through use of the present invention,other techniques allowing the detection of these and other materials byway of physical contact between the material and a sensor element areknown in the art and have in fact been disclosed in prior art patents ofother persons associated with the assignee of the present invention,see, for example, the U.S. Pat. Nos. 5,071,770; 5,045,285 and 4,893,108of Edward S. Kolesar Jr. The Kolesar detection arrangements may involveFourier Transform signal processing of changed electricalresistance-based signals.

Crystal Structure

FIG. 3 in the drawings consists of three crystal structure viewsrelevant to chalcopyrite materials. More precisely the FIG. 3 crystalstructure is relevant to several chalcopyrite materials discussed in thepresent document including the AgGaTe₂ material of example 1 below, theAgGaSe₂ material of example 2, and the AgGaS₂ material of example 3 andthe Silver Gallium Selenide Telluride, AgGa(Se_((1−x))Te_(x))₂ materialof example 6. With slight modification the FIG. 3 structure is alsorelevant to the AgGa_((1−x))In_(x)Se₂ cation material discussed inexample 5 below.

In the FIG. 3 drawings FIG. 3a shows a three dimensional tiltedperspective representation of a quaternary chalcopyrite crystal and FIG.3b shows a face centered view of the FIG. 3a crystal as it appearslooking in a direction parallel with the 001 axis of FIG. 3a andtherefore looking down on the FIG. 3a crystal. FIG. 3c in the FIG. 3views shows the appearance of the FIG. 3a and FIG. 3b crystal from aslightly elevated point lying intermediate the 100 and 010 axes of FIG.3a. The axis 302 in FIG. 3a and the other axes 301 and 303 in the FIG. 3views may be used as guides to identify planes being discussed in thefollowing paragraphs.

Parenthetically the FIG. 3a view of the FIG. 3 chalcopyrite material canitself present visualization and illusion difficulties; it is, forexample, easy to interpret the outline of this drawing as a planarhexagon having slightly tilted sides rather than as the intended threedimensional cube. Realization that the lowermost and uppermost comers inthe FIG. 3a drawing represent lower plane and upper plane end points ofa first major cube diagonal may be helpful in achieving the desiredperspective. Similarly the rightmost and leftmost corners in the FIG. 3adrawing represent lower plane and upper plane endpoints of a second cubediagonal, a diagonal somewhat orthogonal to the first diagonal. A colorrepresentation of the FIG. 3 drawings wherein the colors, according tothe atom composition key at 300, are red, green and yellow respectively,is also helpful in visualization; (such a representation can be madefrom the FIG. 3 drawings with coloring pencils). Representations of thisnature made with the commercial software “MACMOLECULE”™ appear on thefront cover of the periodical publication “MRS Bulletin”, Volume 23number 7, July 1998, Materials Research Society, Warrendale Pa. The “MRSBulletin” drawing is identified as being relevant to a differentchalcopyrite material, Zinc Germanium Phosphide, however the presentchalcopyrites, and indeed many chalcopyrite materials, are of similarcrystal structure.

In the FIGS. 3a, 3 b and 3 c representations, atoms of the four elementscomprising the quaternary chalcopyrite material appear in specificlattice locations and are represented by the different drawing shadingsshown in the FIG. 3a key at 300. Only three atoms are represented inthis key and in the lattice structures since two of the quaternaryelements, either Sulfur and Tellurium or Selenium and Tellurium, arepresent in x and 1−x differential fractional amounts in the quaternaryalloys and therefore cause the FIG. 3a drawing to differ for eachpossible alloy composition if these elements are precisely distinguishedin a drawing. The sites occupied by such Sulfur and Tellurium orSelenium and Tellurium atoms are therefore shown with a single keysymbol 305 in the FIG. 3 drawings and in yellow in the above described“MACMOLECULE”™ representation irrespective of the differential alloycomposition concept.

Considering the FIG. 3 crystal lattice from a crystallography viewpoint,the lattice structure as represented there may be described withreference to planes and axes (identified in bold-faced type to precludeconfusion with drawing element identification numbers herein) in thefollowing manner:

a crystalline cubic lattice 304 located at intersection 306 of the 100,001 and 010 axes 302, 303, 305, the lattice 304 having lattice initialplane faces 308, 310, 312 received in planes defined by each of the100-001, 100-010 and 010-001 axis pairs, having sublattice-definin,lattice mid planes 314, 316, 318 distal to and parallel with the latticeinitial planes 308, 310, 312 respectively and having exterior faceplanes 320, 322, 324 distal to and parallel with each lattice initialplane 308, 310, 312 and each lattice mid plane 314, 316, 318 when viewedalong any of three paths 326, 328, 330 parallel to the 100, 001 and 010axes, paths orthogonal to the 001-010, 100-010 and 100-001 planes,

Gallium atoms 332 located in each sublattice corner of a lattice midplane lying along, the 100 axis 302 parallel with the 001-010 plane,plus

Gallium atoms 334 located at each sublattice center in the latticeinitial and exterior planes 312, 324 lying along the 100 axis 302commencing at the 001-010 plane, plus

Gallium atoms 336 located at sublattice centers of the initial, the midand the exterior planes 310, 316, 322 within a first half 337 of thecube lattice 304, the half located parallel to and adjacent the 001-010plane, along the 100 axis 302, plus

Silver atoms 338 located at each sublattice corner in lattice initialand exterior plane faces 320, 324 disposed along the 100 axis 302parallel to the 001-010 plane, plus

Silver atoms 340 located at sublattice centers of the initial, the midand the exterior planes 310, 316, 322 along the 001 axis within a secondhalf 342 of the cube lattice, a half located parallel to and distal ofthe 001-010 plane along the 100 axis 302, said Gallium and Silver atomsoccupying cation sites in said crystal lattice and comprising an orderedcation sublattice structure, plus

one of differential number-quantity, similarly located, Sulfur andTellurium atoms 343 and differential number-quantity, similarly located,Selenium and Tellurium atoms 343 the differential number-quantity atoms343 being received in planes intermediate the lattice irnitial plane,the lattice mid plane and the lattice external plane in mediate planeslying along each of said 100, 001 and 010 axes, the differentialnumber-quantity atoms 343 being disposed in ordered array in random fillanion lattice patterns paralleling each of the 100-010,100-001 and010-001 planes. Said differential number-quantity Sulfur and Telluriumatoms and said differential number-quantity Selenium and Tellurium atomsoccupying anion sites in said crystal lattice. Said cation sites andsaid anion sites in said crystal lattice comprising in combination achalcopyrite lattice structure.

The relationship of the FIG. 3 crystal with respect to the 100, 001 and010 axes is of course arbitrary but nevertheless helpful for descriptivepurposes as it is not easily visualized. The preceding paragraphs ofgeometric description will of course change if another crystal to axisrelationship such as the unit cell description is selected. Severaloptical parameters of present interest are also represented in the FIG.3 drawings; these include the directions of maximum and minimum|n_(e)−n_(o)| at 347 and 349; the lattice parameter, a, at 351; the caxis or optical axis of the crystal at 353; the anion sublattice at 355in both FIG. 3b and FIG. 3c aid the cation sublattice at 357 in bothFIG. 3b and FIG. 3c.

Considering the FIG. 3 drawings from a crystallography viewpoint,persons skilled in the art will probably recognize that the major atomsin the FIG. 3 structure are arranged in a diamond-like pattern and thatthe differential quantity Sulfur and Tellurium atoms and differentialquantity Selenium and Tellurium atoms 343 are as indicated abovereceived in anion locations of this FIG. 3 pattern. This location is infact believed to be a significant aspect of the present invention sincesuch anion location may be observed to retain the fundamentalarrangement of the crystal, i.e., the Silver and Gallium cation atomstructure, undisturbed. Moreover since it is this fundamental cationorder of a single crystal material which largely determines the opticalproperties of the material, the FIG. 3 crystal predicts that theresulting chalcopyrite material will have characteristics not radicallydifferent from those of the basic alloy, i.e., from the opticalcharacteristics of a Silver Gallium alloy. This is believed to be asignificant advantage of the invention in comparison with present daynonlinear optics trends. The cation sub lattices can be seen as everyother plane of atoms in FIG. 3b and FIG. 3c.

This undisturbed nature of the FIG. 3 crystal is notable in anotherrespect, since certain of the optical materials heretofore consideredfor infrared laser device improvement use do not achieve the undisturbedlattice represented in FIG. 3. In particular the Silver Gallium IndiumSulfide and Silver Gallium Indium Selenide materials of considerablecurrent discussion in this art are altered cation sub-lattice materialsrather than the presently espoused altered anion materials and thereforeprovide optical characteristics different from those of the SilverGallium alloy. In particular it is notable that these materials requirelarger concentrations to achieve a given change in optical propertiesand tend to have greater temperature sensitivity. In addition cationsize differences make it very difficult to maintain optical uniformitywith cation alloying in contrast to near the near same sized anion forwhich optical uniformity is inherently more uniform. This inferioruniformity destroys phase match and accounts for poor performance todate for cation alloy crystals.

FIG. 6 in the drawings shows a representation of the indices ofrefraction for ordinary and extraordinary waves in a negativebirefringence nonlinear uniaxial crystal such as the materials disclosedherein. In the FIG. 6 drawing the outer circle 600 represents the indexof refraction encountered by an ordinary ray of any annular dispositionwhen this ray, within the plane relevant to FIG. 6, passes through acrystal of nonlinear uniaxial optical material. Specific values ofordinary ray refractive index n₀ existing along the circle 600, valuesalso along the X and Z axes 602 and 604, are indicated at 606 and 608.

Inscribed within the circle 600 in FIG. 6 is the ellipse 610representing the index of refraction encountered by an extraordinary rayof any annular disposition when such a rai, within the plane relevant toFIG. 6, passes through a crystal of nonlinear uniaxial optical material.As determined by the elliptical shape of the extraordinary ray indexrepresentation, the specific index of refraction value encountered by aparticular extraordinary ray is dependent on the angular orientation ofthe ray with respect to some axis of the nonlinear crystal. A specificvalue of extraordinary ray refractive index is indicated at 612 for theextraordinary ray 614 directed at the angle θ, 616, with respect to theZ axis 604 in FIG. 6; other n_(e) values are illustrated at 608 and 620in the drawing. The symbol at 616 in FIG. 6 indicates each representedextraordinary ray refractive index along the ellipse 610 is a functionof the angle θ. As indicated at 618, the FIG. 6 drawing is relevant tothe negative birefringence case wherein the n_(o) index is greater thann_(e). As shown in the above identified Springer-Verlag handbook text adrawing somewhat similar to FIG. 6 excepting for its circle beinginscribed within its ellipse can be used for the positive birefringentmaterial case; positive birefringence material is, however, of primarilyacademic interest with respect to the present invention since it isincapable of supporting the beam walkoff-free operating characteristicsof present invention interest.

The FIG. 6 drawing is of assistance in gaining an appreciation for thesignificance of walk off free operation of a laser device according tothe present invention. Generally such walkoff free operation resultsfrom the fact that o-rays propagate in a direction normal to the circle600 or the spherical surface in FIG. 6 drawing as is represented at 624and e-rays propagate in a direction normal to the ellipsoidal surface inFIG. 6-as is shown at 626 for a propagation direction θ. Only at θvalues of zero and ninety degrees do both rays travel in the samedirection. As has been indicated heretofore the achievement of desirablewavelength conversion efficiency in a nonlinear optical device requiresthe maintenance of closely aligned input and output beams within thenonlinear crystal i.e., requires maintenance of close alignment betweenthe beams 120 and 122 in the FIG. 1 drawing. With closely aligned beansenergy transfer between input and output beams of the nonlinear crystal110 is maximized and highest possible energy conversion efficiency isobtained in the nonlinear crystal. Temperature induced crystal dimensionchange is, however, one factor which can disturb this maximum energycoupling relationship in a nonlinear optical crystal. In facttemperature change is often used to tune the birefringence of anonlinear optical material to a desired operating range.

In summarization it is desired for the o-ray and e-rays in the nonlinearcrystal to travel with identical velocities through the crystal. Duringthis travel the paths may have slightly different angular trajectoriesand the difference angle between these paths is identified as thewalkoff angle, (ρ). A mathematical expression relating the quantities ρ,θ, n_(o) and n_(e) appear as equation 2.21 in the Springer-Verlag text.At the zero degree and ninety degree or x axis and z axis locations inthe FIG. 6 drawing the propagation directions are normal to the surfaceof the circle representing the refractive index for the ordinary ray andalso normal to the ellipse representing the refractive index for theextraordinary ray, at other locations an angular difference existsbetween these normals. In fabricating a nonlinear chalcopyrite crystalfor the present uses it is desired to have the crystal faces disposed atsuch angle as to allow rays to enter the crystal face along a normalpath without refraction and while in the crystal to propagate underphase matched energy exchange conditions. In this geometry a minimum ofbeam steering occurs simplifying laser characteristic selection.

Even though the quaternary alloys Silver Gallium Selenide Telluride,AgGa(Se_((1−x))Te_(x))₂, and Silver Gallium Sulfide Telluride,AgGa(Se_((1−x))Te_(x))₂ are the materials of primary focus in thepresent invention, a consideration of related nonlinear chalcopyritematerials, materials indeed usable to make the quaternary alloys andalso nonlinear chalcopyrite materials in their own right, is informativeand provides the basis of several examples presented in this document.These examples comprise what is herein referred-to as the “closelyrelated material” disclosure of the focused-upon quaternary chalcopyritenonlinear optical materials.

In addition to this “closely related material” based disclosure it ispossible to determine certain design properties of the quaternarychalcopyrite alloys of principle focus in the present document (i.e.,Silver Gallium Selenide Telluride, AgGa(Se_((1−x))Te_(x))₂ and SilverGallium Sulfide Telluride, AgGa(Se_((1−x))Te_(x))₂ from knowledge of theproperties of component materials used to fabricate the quaternarymaterials. These component materials are in fact the same relatednonlinear chalcopyrite materials as are disclosed in the “closelyrelated material” disclosure. The following examples, therefore, includematerials having double significance with respect to the focused uponquaternary materials of the present invention; these “closely relatedmaterial” are both component materials and are materials of such closerelationship as to be usable in predicting the characteristics of thequaternary materials. The first of these example materials is a ternarychalcopyrite alloy, an alloy having a positive birefringencecharacteristic, i.e., the material Silver Gallium Telluride, AgGaTe₂.Other of the quaternary fabrication component materials are disclosed insubsequent examples.

EXAMPLE 1, ArgaTe₂

The example 1 AgGaTe₂ ternary alloy bears a special significance to thepresent invention Silver Gallium Selenide Telluride,AgGa(Se_((1−x))Te_(x))₂ and Silver Gallium Sulfide Telluride,AgGa(Se_((1−x))Te_(x))₂ quaternary alloys in that it is believed to haveheretofore been unavailable in such form and quantity as to allow itscharacterization by others working in the nonlinear optical art. Thesuccess of the present inventors in overcoming this obstacle hastherefore been enabling not only with respect to use of the AgGaTe₂alloy itself but has also enabled the significant step of allowingbootstrap characterization of the quaternary alloys Silver GalliuimSelenide Telluride, AgGa(Se_((1−x))Te_(x))₂ and Silver Gallium SulfideTelluride, AgGa(S_((1−x))Te_(x))₂ of present invention interest.

In addition to being a desirable starting component for fabrication ofthe quaternary chalcopyrite alloys of present focus, the intrinsicproperties of the chalcopyrite ternary semiconductor AgGaTe₂ indicate itis in its own right a promising nonlinear optical (NLO) material for usein high average power broadly tunable solid state infrared laser systemsbased on the processes of second harmonic generation (SHG) and opticalparametric oscillation (OPO). The positive birefringence characteristicof Silver Gallium Telluride, AgGaTe₂ precludes a laser device using suchmaterial from achieving advantages achieved with the quaternary negativebirefringence materials, however, the material is otherwise similar andbelieved relevant to the quaternary chalcopyrite materials of presentfocus. AgGaTe₂, for example, provides large second order nonlinearsusceptibility χ, [see N. N. Konstantinkova Yu V.Rud', Sov. Phys.Semicond. 23, 1101 (1989)], a large birefringence, a broad infraredtransparency range, a competitive thermal conductivity, and desirablemechanical properties. Of these properties, adequate birefringence isarguably of high significance for attaining high average power as it maybe used to establish the condition of phase matching to optimize energytransfers between rays in a crystal.

The temperature dependence of the birefringence of AgGaTe₂ has beendetermined for what is believed to be the first time in connection withwork toward the present invention. This temperature dependence isapproximately one third of that reported for AgGaSe₂ and nearly equal to12×10⁻⁶/° K. across the infrared spectrum. More accurate values aredisclosed in the following Table 1 as a function of wavelength and thesame data is shown in graphic form in the FIG. 11 drawing. Thistemperature dependence parameter controls temperature stability andprovides a calibration for temperature tuning in an AgGaTe₂ embodimentof the crystal 110 in FIG. 1; it is also somewhat useful in predictingbirefringence temperature dependence of the quaternary materialsAgGa(S_((1−x))Te_(x))₂ and AgGa(Se_((1−x))Te_(x))₂ of consideration inthe present invention.

TABLE I Temperature Dependence of AgGaTe₂ Birefringence Wavelength inMicrons d(delta n)/dT^(−/° K.) ⁻ 2.0 13.4 × 10⁻⁶ 4.0 12.1 × 10⁻⁶ 6.011.7 × 10⁻⁶ 8.0 11.4 × 10⁻⁶ 10.0 10.8 × 10⁻⁶

As the AgGaTe₂ chalcopyrite crystal structure is non-centrosymmetric,this compound possesses the essential property of a non-zero secondorder nonlinear susceptibility χ which can be quite large. As χincreases rapidly with decreasing band gap, the wavelength conversionefficiency for AgGaTe₂ is significantly larger than that of thestate-of-of-the-art CO₂ laser doubling crystal, AgGaSe₂. The band gapfor AgGaTe₂ is 1.316 electron volt. An experimental value for χ, forAgGaTe₂ has not been reported but in the article by A. G. Jackson, M. C.Ohmer, S. R. Leclair, in Infrared Physics & Technology, 38, 233 (1997) χis estimated by two methods obtaining the values of 170 pm/V and 220pm/V. The traditional Miller rule indicates a value of 344 pm/V. Notablyeven the lowest of these values is a factor of 2.3 larger that that forAgGaSe₂. The relevant figure of merit for conversion efficiency forAgGaTe₂ is near (χ⁽²⁾)²/n³ where n is the nominal index of refraction atenergy levels well below the band gap. This factor has been directlyestimated also by Jackson et al. for AgGaTe₂ and found to be a factor ofthree superior to that for AgGaSe₂.

Another essential property of chalcopyrite materials for presentpurposes is the property of birefringence. Birefringence is a result ofa uniaxial tetragonal distortion from the underlying diamond-likeface-centered cubic structure. This distortion ranges from about zero toten percent and it is usually described by the c/2a ratio, where c and adesignate the latlice constants. The c/2a ratio for AgGaTe₂ has beendetermined to be 1.90 from analysis of x-ray powder patterns, see P.Kistaiah, Y. C. Venudar, K. Sathyanarayana Murthy, Leela Iyengar, and K.V. Krisna Rao, J. Appl. Cryst., 14. 281 (1981). Table III belowdiscloses measured values for the birefringence index of AgGaTe₂ as wehave determined by measurement in connection with the present invention.The table III values are reasonably close to values hypothesized in theliterature including the data reported by R. R. Reddy, Y. NazeerAhammed, in Infrared Phys. & Technology, 36, 825 (1995) where an averageindex of 3.2 is listed without a source reference. The Table III valuesare also close to the value one calculates using a Moss-like trendrelationship as disclosed by Jackson et al. and are near the value of3.0 calculated using reported reflectance measurements.

A near intrinsic infrared transmission spectra has not been reported forAgGaTe₂ as the transmissions of crystals in previous studies, i.e., inB. Tell, J. L. Shay, and H. M. Kasper, Phys. Rev. B, 9, 5203 (1974) andin N. N. Konstantinkova Yu V.Rud′, Sov. Phys. Semicond. 23,1101 (1989)were limited by absorption due to native acceptor defects. Thisextrinsic absorption is so severe that intrinsic absorption has beenstudied largely by reflectivity, see C. Julien, I. Ivanov, A. Khelfa &F. Alapini, M. Guittard, J. of Materials Science, 31, 3315 (1996).However, the range of transparency of AgGaTe₂ can be estimated fromreported values of both the room temperature band gap (1.316 electronvolts, B. Tell, J. L. Shay, and H. M. Kasper, Phys. Rev. B, 9, 5203(1974) and C. Julien, I. Ivanov, A. Khelfa & F. Alapini, M. Guittard, J.of Materials Science, 31, 3315 (1996) and the characteristic frequencyof the highest energy fundamental infrared active phonon, E(LO), 205cm−1. The range can be quite broad, extending from the band edge at 0.91microns to the onset of two phonon absorption processes, at 24.4 microns(onset of strong absorption). The single surface reflection coefficient(R) has been reported in N. N. Konstantinkova Yu V.Rud', Sov. Phys.Semicond. 23,1101 (1989) to have a nearly constant value of 25% from 1to 25 microns. It follows from R that the transmission (T) of anon-absorbing uncoated slab of this material is 60% and the averageindex is nominally 3.0.

FIG. 9 in the drawings discloses a family of values measured inconnection with the present invention for the infrared transmissionspectra of AgGaTe₂. In the FIG. 9 drawing the room temperaturetransmission properties of a sample of 1.21 millimeter thick AgGaTe₂material are represented. In this FIG. the curves a and a′ are obtainedfor the case of o-rays on respectively near infrared and far infraredspectrophotometers. Curves b and b′ are similarly obtained for e-rays.Curves c and c′ provide the reference spectra for these measurements.

We estimate the thermal conductivity of AgGaTe₂ to be 0.8 W/m−° K. byanalysis of its band gap trend within this semiconductor family. Thisvalue is comparable to the value of 1.1 W/m−° K. for AgGaSe₂ reported byJ. Donald Beasley in Applied Optics, 33, 1000 (1994). In addition, Tellet. al. cited above report that AgGaTe₂ has excellent mechanicalproperties concluding that it is the most structurally adequate compoundstudied. Strangely AgGaTe₂ has been largely overlooked in comparison toAgGaSe₂, considering that in principle, it has superior intrinsicoptical properties and comparable mechanical and thermal properties. Theproperties of AgGaTe₂ to be reported herein include the birefringence,the index of refraction, the range of transparency, and the temperaturedependence of the band gap as well as native defect related sub-band gapphotoluminescence and electrical transport properties. These propertiesdefine the potential of this material for the wavelength conversionprocesses of OPO and SHG.

The modern samples reported upon here may be grown by unseeded andseeded horizontal dynamic gradient freeze according to the processreported in the U.S. Pat. No. 5,611,856 of Schunemann et al. Atransparent furnace is utilized to facilitate seeding and low thermalgradients and growth rates typical of those appropriate for otherdifficult to grow chalcopyrites are exploited, see also Peter G.Schunemann and Thomas M. Pollak, MRS Bulletin, 7, 23 (1998). All samplesare considered as grown and three samples from two crystal growths areconsidered extensively; first, an unoriented sample (#2A) of dimensions5.1×5.1×8.3 mm³; second, an oriented sample (#4A) of dimensions 6×5×2.16mm³, where the c-axis is parallel to the long dimension and lying in thesurface plane; and third, sample #4C, a pie-slice shaped prism with anapex angle of 28.2 degrees where the c-axis is parallel to the prismedge defining the apex. One early sample subsequently was fabricatedinto X-ray powder pattern samples. All samples are optically polishedand silvery in appearance. The birefringence measurements are obtainedin a two step process using sample #4A.

Initial sample measurements are taken at the thickness 2.158 mm+−001 andsubsequently thinned to a thickness of 1.210 mm+/−0.001 and themeasurements repeated. This procedure is used so that the order of thepeaks in the polarization interference is properly assigned. The apexangle is chosen to optimize the accuracy of the index measurement for anindex of nominally 3.0. For transport measurements, gold wires areindium soldered to the four corners to provide Hall effect samples inthe Van Der Paaw geometry. The reported fitting parameters for theSellmeier expressions are obtained using a nonlinear fitting routineavailable in the software application Origin ™, where the fittingfunction is user definable.

X-Ray Data

AgGaTe₂ is generally considered to have the chalcopyrite crystalstructure, however, information regarding the diffraction patternexpected from AgGaTe₂ powder samples at room temperature and atmosphericpressure is not known to be reported in the literature. Therefore, thex-ray diffraction pattern obtained at room temperature from AgGaTe₂using the Cu K{dot over (α)} line is reported in Table II below. Theangles and relative intensities of ten of the thirteen lines identifiedin this data agrees well with that expected in a chalcopyrite structure.A reasonably strong line (having relative intensity of 8 out of 100) ata d-spacing of 1.811 and two weaker lines at 2.988 microns and 1.581microns are not, however, identifiable with either the reported orcalculated spectra. Nevertheless, all strong lines (of greater than 20relative intensity) match the anticipated chalcopyrite crystal structurewell. The lattice parameters using the data in Table II are calculatedto be a=6.2786 angstroms and c=11.9637 angstroms for a c/a ratio of1.905. The disclosed value for c/2a agrees to within 0.3% of thepreviously reported values. Eleven lines in the powder pattern areunambiguously indexable in the chalcopyrite pattern.

TABLE II Calculated and Experimental Diffraction Pattern for AgGaTe₂ h k1 d d(Exp) 2θ 2θ(Exp) I(Exp), I(Theo) 1 0 1 5.54 15.96 0, 3 1 1 2 3.5593.562 25.02 25.00 100, 100 1 0 3 3.354 3.363 26.58 26.50 2, 7 2 0 0 3.143.143 28.42 28.40 2, 1 2.988 29.9 1, 0 2 1 1 2.733 2.734 32.76 32.75 3,13 2 1 3 2.292 39.31 0, 2 2 2 0 2.22 2.222 40.63 40.60 27, 25 2 0 4 2.162.166 41.83 41.70 59, 45 1.897 47.95 8, 0 3 0 1 2.062 43.91 0, 4 3 1 21.884 1.888 48.31 48.20 18, 40 3 0 3 1.851 1.857 49.21 49.05 2, 1 2 1 51.816 50.25 0, 1 1 1 6 1.811 1.819 50.39 50.15 11, 11 1.581 58.35 1, 0 32 1 1.723 53.14 0, 2 3 2 3 1.595 57.81 0, 4 4 0 0 1.57 1.568 58.81 58.904, 12 4 1 1 1.511 1.496 61.36 0, 2 3 3 2 1.436 64.91 0, 7 0 0 8 1.48862.43 62.05 2, 2 3 2 5 1.406 66.52 0, 2 3 1 6 1.403 66.64 0, 8 3 0 71.32 71.49 0, 1 4 1 5 1.283 73.87 0, 2 4 2 4 1.27 74.75 0, 13 4 3 11.249 76.22 0, 2

Birefringence

The birefringence of AgGaTe₂ has been directly measured using thepolarization interference method reported in D. W. Fischer and M. C.Ohmer, P. G. Schunemann and T. M. Pollak, J. Appl. Phys., 77,5942 (1995)and the optimally oriented sample #4A. This data is reported here inwhat is believed to be the first time in patent format. Thebirefringence is found to range from a near band edge value of 0.038 at1.3 microns to a value of 0.017 at 15 microns. The data is plotted inFIG. 7 herein and listed in Table II. It should be noted that thesevalues may represent a lower bound since free carriers of either signcan lower the value significantly, as is reported in D. W. Fischer andM. C. Ohmer and J. E. McCrae, J. Appl. Phys., 81, 3579 (1997). ASellmeier-like dispersion equation is used to fit experimental values,following the procedure disclosed by Gorachand Ghosh, in applied Optics,37, 1205 (1998) where the birefringence (b) is given by,

b=Iλ²/(λ²−C′)+Jλ²/(λ²−F′),  (12)

and λ is the wavelength in microns. This physically realistic functionalform is usually chosen as the parameters C′ and F′ parody the parametersC and F for the Sellmeier fit to the corresponding data for the averageindex of refraction. Additionally, they can be approximately correlatedrespectively with the band gap energy and the energy of the onset ofphonon absorption. The fitting values for the parameters, I, C′, J, andF′ are found to be respectively, 0.01939, 0.73191, 0.11241, and 7227.84.The oscillator energy equivalent of C′, (E_(o)=1.2398/C′^(½)), is 1.449electron volt and the corresponding phonon oscillator wavelength,(F′^(½)), is 85 microns. Ghosh in the afore cited reference relates E.directly to the band gap. However S. H. Wemple and M. Didomenico, inPhys. Rev. B, 3,1338 (1971), while agreeing that it is linearly related,indicate that the relationship is given by E_(o)=1.5 E_(G) or E_(G)=0.97electron volt. In this case, Ghosh's procedure is closer to the actualvalue of 1.356 electron volt. The fit of Equation 12 to our data isshown in FIG. 7.

FIG. 7 also compares the birefringence of similar members of thechalcopyrite family. As this FIG. 7 comparison shows, AgGaSe₂ and AgGaS₂have large negative birefringences in the infrared wavelengths and theyare broadly phase matchable. The birefringence of AgInSe₂ is positivelybirefringent for all wavelengths but the magnitude is too small to beuseful for phase matching. Our measurements indicate that AgGaTe₂ isalso positively birefringent for all wavelengths as is discussed laterand its birefringence is about a factor of 4 larger than that of theAgInSe₂ material discussed in example 4 below. The AgGaTe₂ birefringenceis nearly exactly equal to that of CdSe, its near binary analog which isphase matchable. In order to confidently assess whether AgGaTe₂ is phasematchable, not only the birefringence but the dispersion must be knownaccurately.

Refractive Index

The extraordinary and the ordinary refractive indices n_(o) and n₀ ofAgGaTe₂ listed in Table III are directly measured between wavelengths of3.0 to 5.0 microns by the minimum deviation angle method disclosed byDavid E. Zelmon, David L. Small, and Ralph Page, in Applied Optics,37,4933 (1999). Sample #4C was used for these measurements. The sign ofthe birefringence is established by analyzing the polarization of therefracted beams. This is the first patent document report of theseparameters for AgGaTe₂, although an average value of 3.2 has, as notedabove, been previously listed in a compendium of indices without asource reference, see R. R. Reddy, Y. Nazeer Ahammed, Infrared Phys. &Technology, 36, 825 (1995).

Parenthetically it may be noted that the believed first patent reportingof Table III values in the present document, and the similar firstpatent reporting of other characteristics of the AgGaTe₂ materialherein, are of notable significance with respect to the focused-uponquaternary alloys Silver Gallium Selenide Telluride,AgGa(Se_((1−x))Te_(x))₂, and Silver Gallium Sulfide Telluride,AgGa(Se_((1−x))Te_(x))₂ in a somewhat indirect but neverthelessimportant sense. Applicants believe for example that they are the firstto have available for detailed measurement such quantities of theternary chalcopyrite AgGaTe₂ material as can provide the array ofdetailed characteristics of this material disclosed herein. Heretofore,therefore, it has been impossible to accomplish a “closely relatedmaterial” disclosure, such as the present, for any purpose includingestablishment of a relationship with the focused-upon quaternarychalcopyrite alloys Silver Gallium Selenide Telluride,AgGa(Se_((1−x))Te_(x))₂, and Silver Gallium Sulfide Telluride,AgGa(Se_((1−x))Te_(x))₂ i.e., impossible because usable quantities ofthe ternary “closely related material” were not heretofore available forcharacterization.

TABLE III Extraordinary and Ordinary Indices of Refraction for AgGaTe₂in the Mid-Infrared □ □ (microns) n_(e) n_(o) b = n_(e)-n_(o) 3.03.00473 2.98592 .01881 3.5 2.99750 2.98035 .01715 4.0 2.99337 2.97618.01719 4.5 2.98939 2.97335 .01604 5.0 2.98677 2.97065 .01612

The data set in Table III is based on early, somewhat less than optimumquality, AgGaTe₂ crystals. The dispersion characteristic of the materialmust be accurately known to determine phase matching angles. Thedirectly measured b is 15-20% higher that that determined by the lessaccurate method of differencing the indices but is nominally in goodagreement. It should be noted that these measurements were obtained ontwo different samples and the analysis in either case ignored freecarrier, rotary power and polarization sensitive precipitationscattering effects. As noted in the published article of D. W. Fischer,M. C. Ohmer and J. E. McCrae appearing in the Journal of Applied PhysicsVolume 81, at page 3579 (1977) free carriers of either type can decreasethe birefringence of a given sample.

Sellmeier expressions for the indices may be obtained using a two stepmethod employing both the index and birefringence data, the n and bdata. First, a Selimeier fit to the average index data from 3-5 micronsis obtained using data listed in Table III. The first estimate for theparameters is based on a knowledge of the band gap energy, thecharacteristic phonon energies and the experimental indices in order toprovide the usual somewhat physically realistic expression. Anon-physical highly accurate analytical expression obtained by fittingour b values from nominally 1-15 microns and our initial Sellmeier forthe average index may be used to construct a data file for n_(e) andn_(o). This constructed file may then be used to obtain the Sellmeierexpressions given in Equations 13 and 14. The resulting parameters aredisclosed in Table IV below.

n_(e) ²=A_(e)+B_(e)λ²/(λ²−C_(e))+D_(e)λ²/(λ²−F_(e))  (13)

n_(o) ²=A_(o)+B_(o)λ²/(λ₂−C_(o))+D_(o)λ²/(λ₂−F_(o))  (14)

TABLE IV Parameters for Three Term Sellmeier Expressions for AgGaTe₂Parameter n_(e) ² n_(o) ² A 8.63014 8.57181 B 0.31139 0.2582 C 2.286162.38844 D 1.61668 1.5502 E 961 961

Equations 13 and 14 are also shown plotted in FIG. 8 herein. The indexdata from Table II are also plotted in FIG. 8. The slightly smallerbirefringence in the index data mentioned before is apparent. Theaverage index from our Sellmeier expression is also shown in FIG. 8 as adashed line. FIG. 8 shows that the indices constructed from theexperimental birefringence and the Sellmeier expression for the averageindex are described very well by equations 13 and 14 as the overlay isquite precise.

Transmission and Absorption

The spectral transmission curves for an AgGaTe₂ sample #4A are shown inFIG. 7. The thickness of the sample is 1.21 mm. The expected extremelybroad transmission range spectra was obtained. In this sample the peaktransmission is 56% for o-rays, a value within 4% of that expected forno optical loss for a material with an index 3.0. The range nominallyextends from 1 to 21 microns from the band edge to the onset of twophonon absorption. A band tail extends for 5-6 microns beyond the bandedge for both polarizations, this is evidence that the transmission ofthe sample is limited by deep levels as is typical for compoundsemiconductors. This defect related sub-bandgap near edge absorption isobserved in all of our AgGaTe₂ samples. This absorption band does notfreeze out, persisting instead to low temperatures. The sub-bandgap nearedge absorption band for sample #4A obtained at a temperature of 4.5 °K. is analyzed. Deconvolution occurs into two Gaussian indicated peakpositions at 0.946 and 1.00 electron volts where the peak at 0.946electron volts was the most intense. Even though the initial crystalsstudied have far from optimum crystal quality, their transmissionspectra indicates that it should be possible to produce crystals with aremarkably broad transmission range with a near intrinsic transmission.

FIG. 9 in the drawings shows the measured room temperature transmissionproperties of a sample of 1.21 millimeter thick AgGaTe₂ material. Inthis FIG. the curves a and a′ are obtained for the case of orays onrespectively near infrared and far infrared spectrophotometers. Curves band b′ are similarly obtained for e-rays. Curves c and c′ provide thereference spectra for these measurements.

By accident the AgGaTe₂ sample #4A and another sample, sample #2A havedifferent thermal histories. Sample #2A experienced rapid cooling duringthe final growth step while sample #4A cooled more slowly. Near infraredimaging of sample #2A does not show finely divided second phaseoptically scattering precipitates as are typically observed in AgGaSe₂.However, the usual precipitate described by G. C. Catella and D. Burlagein Materials Research Society (MRS) Bulletin, 23,28 (1998) is observedin sample #4A. As this precipitate is the source of scattering loss, itmay partially explain the low transmission of sample #4A. Defect relatedsubbandgap near edge absorption is observed in our AgGaTe₂ samples asshown in FIG. (3) for sample #4A. The FIG. 4 data is taken at atemperature of 4.5K. After baseline subtraction, a Gaussian is fitted tothe FIG. 3 data peak, and then subtracted from the parent spectrum. Thenew spectrum reveals an additional peak, which is also fitted to aGaussian and subtracted. The Gaussians, the spectrum, and bothsubtracted spectra are shown in FIG. (3). The peaks have energies of0.946 electron volt and 1.00 electron volt. This data indicates thetransmission of our samples is extrinsically limited by native defects.

The optical and electrical properties of compound semiconductors aregenerally controlled by native point defects such as vacancies oranti-sites, and the chalcopyrite semiconductors are no exception,particularly with regard to cation vacancies. This is due to a sizemismatch between the two cations in the ordered cation sub-lattice.These defects are usually donors or acceptors which can act as absorbingcenters via photoionization and they are the source of thermallyactivated carriers that cause free carrier absorption.

The temperature dependence of the bandgap may be determined from ananalysis of the band gap absorption edge. Specifically, the secondderivative of the band edge may be taken, and the peak marking theedge's point of inflection then used as an indicator of the band edge.This estimate of the bandgap is found to vary from 0.99 electron volt atroom temperature to an extrapolated value of 1.10 electron volt at zerodegrees Kelvin (K). Typically, due to absorption by deep levels, theband gap obtained by analysis of the absorption edge underestimates theactual gap as is true in this case. Electroreflectance measurements asdisclosed in B. Tell, J. L. Shay, and H. M. Kasper, Phys. Rev. B, 9,5203 (1974), for instance, indicate that the actual room temperature gapis 1.316 electron volt. However, such analysis does yield an accuratemeasure of the relative change of the bandgap with temperature. Theaverage dE/dT is fourd to be −4.7×10⁻⁴ electron volt/° K. in thetemperature range from 150° K. to 300° K. This compares favorably withthe previously reported value of N. N. Konstantinkova Yu V.Rud' in Sov.Phys. Semicond. 23,1101 (1989) of dE/dT of −3.6 10⁻⁴ electron volt/° K.over the larger range of 8020 K. to 350° K. For such a range, we wouldobtain a similar value as a result of saturation. This parameter is ofinterest as it largely controls the temperature dependence of n and theb through the band gap dependence of the first term of Equation 12 andthe second term in Equations 13 and 14. Fits can be made to both theVarshni¹⁹ and Cody²⁰ analytical expressions, the expressions disclosedin Y. P. Varshni, Physica 34, 149 (1967) and in G. D. Cody, inHydrogenated Amorphous Silicon, edited by J. I. Pankove, Semiconductorsand Semimetals vol. 21, Pt. b (Academic, New York, 1984), Chap. 2, pp.11-79. The Varshni expression given below provides the best fit.

E_(g)(T)=1.20−(0.001T²)/(T+607)  (14)

In this equation E_(g)(T) is the band gap in electron volt and T is theabsolute Kelvin temperature.

Photoluminescence Data

The low temperature photoluminescence (PL) of AgGaTe₂ has beeninvestigated using the technique reported by H. M. Hobgood, T.Henningsen, R. N. Thomas, and R. H. Hopkins, M. C. Ohmer, W. C. Mitchel,D. W. Fischer, S. M. Hegde, and F. K. Hopkins in J. Appl. Phys. 73, 4030(1993). Although this is believed a direct gap material, band to bandphotoluminescence is not observed in our samples. However, sub-band gapphotoluminescence is observed and typical spectra are shown in FIG. 10herein. In these initial measurements polarization of the emission isnot analyzed nor is the polarization of the exciting laser controlled.Additionally sample 2A is oriented at an unknown arbitrary angle. Sample4A is oriented such that the c-axis is parallel to the plane of thesurface of the sample. This sub-band gap photoluminescence is spectrallyquite broad, peaking at 0.8 electron volt and similar in appearance tothat observed in many other compound semiconductors. As the crystals arenominally undoped and grown from ultra-pure materials, thisphotoluminescence is most likely due to native defects such as vacanciesor antisites typically observed in other chalcopyrites.

Transport Data

Sample #2A#2, of dimensions 5×5×2.5 mm³ is provides temperaturedependent Hall effect data. Sample #2A#2 is cut from sample #2A, asample having the transmission shown in FIG. 9. This analysis indicatesthe sample is p-type as grown; this has been observed previously as isreported by B. Tell, J. L. Shay, and H. M. Kasper in Phys. Rev. B, 9,5203 (1974) and also by N. N. Konstantinkova and Yu V. Rud' in Sov.Phys. Semicond. 23,1101 (1989). The carrier concentration, resistivity,and mobility at 300° K. are respectively, 1.4×10¹⁴ cm⁻³, 3800 ohm-cm,and 11 cm/Vs. Analysis of the high temperature region for both theresistivity and the carrier concentration curves provides an activationenergy of 0.30 electron volt. The low temperature region showssignificant hopping conduction. B. Tell, J. L. Shay, and H. M. Kasper inPhys. Rev. B, 9, 5203 (1974) reported that their AgGaTe₂ crystals asgrown varied from moderately conducting p-type to high resistivityp-type with carrier concentration to 10¹⁷ cm⁻³ and Hall mobilities inthe range of 20-40 cm/Vs. N. N. Konstantinkova and Yu V. Rud′ in Sov.Phys. Semicond. 23,1101 (1989) report, for as grown materials, p-typebehavior characterized by an activation energy of 0.15 electron volt insamples with a carrier concentration in the range of 1-5×10¹³ cm⁻³ andHall mobilities in the range of 5-8 cm/Vs. They also report significanthopping. All results are in reasonable agreement except with regard toactivation energy. This may indicate that different deep native acceptordefects are being observed or the growth processes have introduceddifferent p-type impurities. The crystals in all cases are grown fromstoichometric melts formed from elemental starting materials. However,Konstantinkova and Rud used graphite coated silica boats while ourcrystals are grown in pyrolytic Boron nitride-coated graphite boats.

AgGaTe₂ Phase Matching at Room Temperature

Utilizing Equations 13 and 14 for the indices of AgGaTe₂, thepossibility of Type I phase matching for the processes of SHG anddegenerate OPO generation can be investigated under a plurality ofdifferent conditions. These conditions include investigation at roomtemperature, at elevated temperature and for AgGaTe₂—AgGaSe₂ mixedcrystals at room temperature. It is found that AgGaTe₂ does not phasematch at room temperature for the usual CO₂ and two micron pump lasers.This is because the birefringence is not large enough to compensate forthe dispersion predicted. For this case for SHG of 10.6 microns,birefringence is 0.0176 and a value of 0.022 or more is required forphase matching, a shortfall of 0.0044 or 25%.

Phase matching is, however, realizable in AgGaTe₂ at slightly elevatedtemperatures i.e., through use of the mechanisim of temperature tuning.This is because the magnitude of the birefringence generally increasessignificantly with temperature for both the cases of positive andnegative birefringence. This characteristic has been described by M. C.Ohmer and R. Pandey in MRS Bulletin, 23,16 (1998) and by N. P. Barnes,D. J. Gettemy, J. R. Hietanen, R. A. Iannini in Appl. Opt. 28, 5162(1989). Temperature tuning to obtain the necessary birefringence hasbeen previously exploited by Andreev et. al. as is reported in thearticle by Y. M. Andreev, A. N. Morozov, A. V. Sosin, and G. S.Khmelnitskii, Sov. J. Quantum. Electron. 14, 1024 (1984) and by Bahr et.al. as reported in an article by G. C. Bhar, S. Das, and U. Chattergee,P. K. Datta, and Yu. M. Andreev in App. Phys. Lett. 63,1316 (1993). Themagnitude of db/dT for AgGaTe₂ is found to be a value of +11×10⁻⁶/° K.at 3.39 microns. Using this value indicates that AgGaTe₂ will phasematch at 10.6 microns at a temperature of 300° C.

The fundamental optical properties of AgGaTe₂, a nonlinear opticalsemiconductor are therefore reported here. These properties includebirefringence, indices of refraction, infrared transmission, and thetemperature dependence of the band gap. The average index forwavelengths greater than several microns is found to be 3.0. Thebirefringence is found to be rather large and to range from a near bandedge value of 0.038 at 1.3 microns to a value of 0.017 at 15 microns.Additionally, native defect related sub-band gap absorption,photoluminescence and electrical transport properties have been studiedin nominally undcped p-type crystals. An activation energy associatedwith these defects is determine to be 0.37 electron volt and thecorresponding photoluminescence and absorption data shows respectively abroad asymmetric emission band centered at 0.8 electron volt and twobands at 0.95 electron volt and 1.01 electron volt, the former being themost intense. The measured properties are utilized to assess thepotential of AgGaTe₂ for the wavelength conversion processes of opticalparametric oscillation and second harmonic generation.

Room temperature wavelength versus refraction index curves for theAgGaTe₂ material, are shown in FIG. 11a of the drawings herein. Thesecurves relate somewhat to FIG. 2 in the drawings and illustrate materialproperties from a different perspective. One disadvantage of the AgGaTe₂material for present purposes is, for example, apparent in the FIG. 10acurves in that second harmonic generation operation is not possible atthe wavelength of the 10.6 micron carbon dioxide laser line; this isbecause of the excessively large dispersion of the index of refractionoccurring in FIG. 10a and the fact this cannot be compensated since thebirefringence is too small, λ≈170 pm/v. The improvement achieved throughinclusion of a near optimum amount of Selenium to form a quaternaryalloy Silver Gallium Selenide Telluride is apparent in FIG. 11b and isdiscussed under the heading of example 6 below.

EXAMPLE 2, AFGaSe₂

Fabrication of a first one the two quaternary alloys of present focus,i.e., fabrication of Silver Gallium Selenide Telluride,AgGa(Se_((1−x))Te_(x))₂, may in fact commence with the above disclosedAgGaTe₂ ternary chalcopyrite material as a first component and use theternary chalcopyrite AgGaSe₂ material as a second component. Alternatelysuch fabrication can also commence with the four component elements ofthe quaternary AgGa(Se_((1−x))Te_(x))₂ alloy. In either of theseinstances, the horizontal pulling method disclosed in the aboveidentified U.S. Pat. No. 5,611,856 is deemed an appropriate quaternarymaterial fabrication process. In view of the former or ternary alloypossible origination of the quaternary alloys, a discussion of theternary alloy, AgGaSe₂, component as a part of the “closely relatedmaterials” disclosure appears appropriate. The present Example 2 is usedfor this purpose.

Several of the previously discussed drawings in the present patentdocument are relevant to multiple chalcopyrite materials and thereforeare also either generally or specifically descriptive of this presentexample second component ternary alloy, AgGaSe₂ material. This, multiplerelevance is appropriate for example to the FIG. 1 use apparatus drawing(especially since AgGaSe₂ is noted to be the current state of the artwavelength changing material for use in FIG. 1 type apparatus). Thecrystal structure shown in FIG. 3 is also relevant to the secondternary, AgGaSe₂, and the birefringence properties discussed inconnection with FIG. 6 in the drawings also have relevance to thismaterial. Moreover the birefringence and wavelength relationships shownin the FIG. 7 drawing also include curves relevant to this secondternary, AgGaSe₂, alloy.

A plurality of additional properties of the second ternary, AgGaSe₂,alloy are also disclosed in the above referred-to Springer-Verlaghandbook of optical materials, see especially section 3.1.23, the sevenother references to this material in the “subject index” of theSpringer-Verlag handbook and the identified publication “references”disclosed commencing at page 197 of the 1991 Springer-Verlag handbooktext. Further properties of the AgGaSe₂ material and its growth are tobe found in articles appearing in the above identified “MRS Bulletin”,Volume 23 number 7, July 1998 and in the publications identified inthese articles. As is implied by these recitations of properties, theAgGaSe₂ chalcopyrite alloy is also a viable material in its own rightfor end use in a nonlinear optical crystal laser device.

Notably the AgGaSe₂ alloy provides negative birefringencecharacteristics and is therefore a more significant material foremployment, as either an end use crystal or as a component material inpresent invention wavelength changing crystals than are some othermaterials. The band gap for AgGaSe₂ is 1.83 electron volts; this is incomparison with the band gap of 1.316 electron volts for the aboverecited example 1 AgGaTe₂ material. A thermal conductivity value of 1.1W/m−° K. has been reported for AgGaSe₂ by J. Donald Beasley in AppliedOptics, 33, 1300 (1994). The temperature differential of birefringence,db/dT, for AgGaSe₂ has been reported by N. P. Barnes, D. J. Gettemy, J.R. Hietanen, R. A. Iannini in Appl. Opt. 28, 5162 (1989) to haeve avalue of −31×10⁶/° K. at 3.39 microns. Other properties of AgGaSe₂ arealso reported by J. L. Shay and J. H. Wernick in Ternary ChalcopyriteSemiconductors: Growth Electronic Propertes, and Applications (Pergamon,New York, 1975). Additional data regarding characteristics of theAgGaSe₂ alloy, including the source data of the FIG. 7 curve for thisalloy, are available from the earlier work of Boyd et al. as publishedin G. D. Boyd, H. M. Rasper, J. H. McFee and F. G. Storz Institute ofElectrical and Electronic Engineers Journal of Quantum Electronics 8(12) (1972) page 900.

The phase matching angle for AgGaSe₂ is 54.4 degrees for a 10.6micrometers pump and its λ₃₆ ⁽²⁾ is 66 pm/V; these values are reportedin the Springer-Verlag Handbook of Nonlinear Optical Crystals. Thisangle of 54.4 degrees is, however, considerably smaller than the idealphase matching angle of 90 degrees for present invention purposes. Thevalue of the angle it for a 19 percent Tellurium addition to thisAgGaSe₂ is, however, a much better value of 86.2 degrees. The figure ofmerit for conversion efficiency for the 19 percent Tellurium compositionis moreover found to be a factor of 1.5 greater than that of AgGaSe₂,considering only the larger angle θ; its λ₃₆ ⁽²⁾ is also larger as ithas a smaller band gap. These considerations suggest the addition of aselected amount of Tellurium to the AgGaSe₂ material of the presentexample can be of significant benefit for present invention purposes.This benefit is considered in greater detail in Example 6 below.

To conclude this example, the conversion efficiency for AgGaSe₂, themost widely used infrared nonlinear optical crystal, the crystalpresently considered to be the state-of-the-ari: CO₂ laser doublingcrystal, is limited since its birefringence is not optimal and itsnonlinear properties are also sub-optimal. This results in a phasematching angle which does not use effectively the availablenonlinearity, causes excessive walk-off of the signal and pump beams andprovides a low conversion efficiency in view of its λ² value of 66 pm/V.By alloying wit1h an appropriate amount of Tellurium as espoused in thepresent invention and as discussed in example 6 below, the birefringencecan be tuned to a near ideal value for a given application that bestuses the intrinsic nonlinearity and maximizes the useful length of thecrystal.

AgGaSe₂ has been studied extensively and is available commercially.Table V below shows parameters for a three term Sellmeier Expression forAgGaSe₂ as disclosed in the published article of G. C. Bhar appearing inthe journal Applied Optics at volume 15, page 305 (1976). Similar valuesand additional details also appear in table 2.1 of Chapter 2 in the wellknown reference text “Handbook of Thermo-Optic Coefficients of OpticalMaterials With Applications” By Gorachand Ghosh published by AcademicPress of New York, Boston and San Diego. The technical articles “CrystalGrowth and Optical Properties of AgGaS₂ and AgGaSe₂” by G. G. Catellaand David Burlage, “Properties of Dopants in ZnGeP₂, CdGeAs₂, AgGaS₂anti AgGaSe₂” by B. H. Bairamov, V. Yu. Rud, and Yu. V. Rud and“Nonlinear Frequency Conversion Performance of AgGaSe₂, ZnGeP₂ andCdGeAs₂” by P. G. Schunemann, K. L. Schlepler and P. A. Budni allappearing in the July 1998 issue of the “MRS Bulletin” (MaterialsResearch Society, Warrendale , Pa., 15086), also provide detailsregarding the AgGaSe₂ material and include extended lists of additionalreferences. Additionally a growth process for AgGaSe₂ material isdisclosed in the U.S. Pat. Nos. 5,475,526 and 5,355,247 of Robert L.Byer et al., patents which are assigned to The Board of Trustees of theLeland Stanford, Jr. University of Stanford, Calif.

TABLE V Parameters for Three Term Sellmeier Expressions for AgGaSe₂Parameter n_(e) ² n_(o) ² A 5.2912 4.6453 B 1.397 2.2057 C 0.2845 0.1879D 1.9282 1.8377 E 1600 1600

EXAMPLE 3, AGaS₂

A fabrication of the remaining of the two quaternary alloys of presentfocus, i.e., a fabrication of the AgGa(Se_((1−x))Te_(x))₂ sulfide alloy,may also in fact commence with the above disclosed AgGaTe₂ ternarychalcopyrite material as a first component and use the ternary AgGaS₂Silver Thiogallate chalcopyrite material as a second component.Alternately this quaternary fabrication can also commence with the fourcomponent elements of the quaternary alloy. In either of theseinstances, the horizontal pulling method disclosed in the aboveidentified U.S. Pat. No. 5,611,856 is also deemed an appropriate alloyfabrication process. In view of the former or ternary alloy possibleorigination of the quaternary alloys, a discussion of the ternary alloy,AgGaS₂, component as a part of the “closely related materials”disclosure again appears appropriate. The present Example 3 is used forthis second quaternary from second ternary alloy discussion purpose.

Several of the previously referred-to drawings in the present patentdocument are also either generally or specifically descriptive of thispresent second quaternary from second ternary alloy example. The FIG. 1,FIG. 4 and FIG. 5 use drawings are of course relevant to this alloydiscussion from both the end use and component use of AgGaS₂ viewpoints.The crystal structure shown in FIG. 3 and the birefringence propertiesdiscussed in connection with FIGS. 6 and 7 also have relevance to aAgGaS₂ discussion. Generally the substitution of Sulfur for Selenium inchalcopyrite alloys has the effect of changing the favored portion ofthe electromagnetic spectrum for the material. The presence of Sulfur inlieu of Selenium with respect to the AgGa(Se_((1−x))Te_(x))₂ andAgGa(Se_((1−x))Te_(x))₂ quaternary materials for example has the effectof shifting the crystal usable wavelength band slightly toward thevisible region.

A plurality of additional properties of the ternary, AgGaS₂, alloy arealso disclosed in the above referred-to Springer-Verlag handbook ofoptical materials, see especially section 3.1.21, the seven otherreferences to this material (some of multiple pages length) in the“subject index” of the Springer-Verlag handbook and the “references”disclosed commencing at page 197 of the 1991 handbook text. Furtherproperties of the AgGaS₂ material and its growth are to be found inarticles appearing in the above identified “MRS Bulletin”, Volume 23number 7, July 1998 and in the publications identified in thesearticles. One of these articles “Crystal Growth and Optical Propertiesof AgGaS₂ and AgGaSe₂” by G. C. Catella and D. Burlage appearing at page28 has been referred-to above and appears especially relevant. Notablythe AgGaSe₂ alloy also provides negative birefringence characteristicsand is therefore a desirable end use or component material in presentinvention wavelength changing crystals. Additional data regardingcharacteristics of the AgGaS₂ alloy, including the source data of theFIG. 7 curve for this alloy, are available from the work of Boyd et al.as identified in example 2 above.

By way of concluding the present example 3, the conversion efficiencyfor AgGaS₂ is also limited as neither its birefringence nor itsnon-linear properties are optimal for such service. This results in aphase matching angle which does not use effectively the availablenonlinearity, causes excessive walk-off of the signal and pump beams andprovides a low conversion efficiency in view of its low χ² value of 22pm/V. By alloying with an appropriate amount of Tellurium as espousedherein, the birefringence can be tuned to the ideal value fir a givenapplication that best uses the intrinsic nonlinearity and maximizes theuseful length of the crystal. Again Te additions have not previouslybeen exploited as, strangely, Tellurium has been largely overlooked incomparison to the similar elements Indium, Selenium and Sulfur whosecompounds are available commercially.

AgGaS₂ has also been studied extensively and is available commercially.Table VI below shows parameters for a three term Sellmeier Expressionfor AgGaS₂. These values also appear in the published article of G. C.Bhar in the Journal Applied Optics at volume 15, page 305 (1976).Similar values and additional details also appear in the table 2.1 ofChapter 2 in the above identified and familiar “Handbook of Thermo-OpticCoefficients of Optical Materials With Applications” text by GorachandGhosh published by Academic Press of New York, Boston and San Diego.

TABLE VI Parameters for Three Term Sellmeier Expressions for AgGaS₂Parameter n_(e) ² n_(o) ² A 4.0172 3.6280 B 1.527 2.1686 C 0.1310 0.1003D 2.1699 2.1753 E 950 950

EXAMPLE 4, AgInSe₂

It is, of course, possible to achieve birefringence and phase matchingoperation in mixed crystals of the types focused-upon in the presentinvention i.e., through use of the quaternary materialsAgGa(Se_((1−x))Te_(x))₂ and AgGa(Se_((1−x))Te_(x))₂ where x is chosen toprovide the desired room temperature birefringence. One example relevantto this quaternary material approach to birefringence and phase matchedoperation and employing the related indium cation quaternary material,Ag(Ga_(x)In_((1−x))Se₂ , a material whose properties are currentlyknown, is considered in Example 5 disclosed later herein. In the presentexample 4 another indium inclusive material, a material usable as afabrication component of the example 5 AgGa_(x)In_((1−x))Se₂ material isconsidered. The AgInSe₂ material of present consideration has inadequatebirefringence as a ternary material for use in a wavelength conversionprocess and is of primary interest as the cation component of thequaternary Indium material. Other properties of this present exampleAgInSe₂ material are also known including indices values which arereported in the Institute of Electrical and Electronic Engineers Journalof Quantum Electronics, volume QE8, page 900 (1972) authored by C. D.Boyd, H. M. Kasper, J. H. McFee and F. G. Storz.

Several of the previously discussed drawings in the present patentdocument are also either generally or specifically descriptive of thisAgInSe₂, cation alloy. The FIG. 1, FIG. 4 and FIG. 5 laser deviceutilization drawings are of course relevant to this alloy in itsincorporated as a component usage. The crystal structure shown in FIG. 3has relevance to the AgInSe₂ alloy and the birefringence propertiesdisclosed in FIG. 7 include a curve descriptive of this material.Generally a presence of the element Indium in a chalcopyrite crystalresults in modification of the cation lattice of the crystal and somealteration of the non cation included material crystal properties. Inthe present instance such a modification of the cation lattice withIndium reduces the bandgap of the material, increases its nonlinearproperties and tunes its birefringence to attain non critical phasematching. The use of Indium cation material in a chalcopyrite crystalis, however, generally less effective in achieving a given degree ofcharacteristic modification than is use of an anion material such as theTellurium espoused in the present invention; for this reason largervalues of x in the chemical formula are generally appropriate for acation material than for an anion material.

AgInSe₂ is positively birefringent for all wavelengths, however, themagnitude of this birefringence b is too small to be useful for phasematching operation of a nonlinear laser device; this birefringence is infact about one fourth of that for the example 1 AgGaTe₂ material. Thepositive birefringence of course also precludes non critical phasematching and beam vwalk off free operation in such a laser device.Additional data regarding characteristics of the AgInSe₂ alloy,including the source data of the FIG. 7 curve for this alloy, areavailable from the work of Boyd et al. identified in example 1.

AgInSe₂ has also been studied extensively and is available commercially.Table VII below shows parameters for a three term Sellmeier Expressionfor AgInSe₂. Sellmeier values, and additional details relating toAgInSe₂ also appear in the table 2.1 of Chapter 2 in the aboveidentified and familiar “Handbook of Thermo-Optic Coefficients ofOptical Materials With Applications” text by Gorachand Ghosh publishedby Academic Press of New York, Boston and San Diego.

TABLE VII Parameters for Three Term Sellmeier Expressions for AgInSe₂Parameter n_(e) ² n_(o) ² A 5.7110545 5.5429671 B 1.2818184 1.4313841 C0.5574472 0.4543274 D 0.7787102 0.7787102 E 900 900

EXAMPLE 5, AgGa_((1−x))In_(x)Se₂

In a technical article appearing in Applied Physics Letters 63, 1316(1993) G. C. Bhar, S. Das, U. Chattergee, P. K. Datta, and Yu. M.Andreev disclose work with the quaternary Indium chalcopyrite alloyAgGa_(1−x))In_(x)Se₂ and use of this material as a nonlinear opticalcrystal. This work, moreover, includes noncritical phase matchingapplications in the infrared wavelength range of 0.8 to 13 microns anduse of the material in second harmonic generation and optical parametricoscillation applications employing the 10.6 micron carbon dioxide line.Indium contents in the range of thirty to forty percent are included inthis work. This quaternary Indium alloy is reported in this work to havegood properties including strong nonlinear coupling, wide transmissionrange, wide phase matching range and low energy absorptioncharacteristics. There appears, however, some question as to thecommercial availability of this material in sufficient quality andquantity to support its extensive use at the time of present documentpreparation.

Additionally U. Simon, F. K. Tittle and L. Goldberg, in work reported inthe periodical Optical Letters 18 (1993) at page 1931, have determinedaccurate refractive index data for the AgGa₍₁ x)In_(x)Se₂ chalcopyritecrystal; this work is based on crystals of sufficient size and qualityhaving been made available from the former Soviet Union.

The optical properties of the AgGa_((1−x))In_(x)Se₂ chalcopyritematerial, at least such properties as are enabling of its use innonlinear applications relating to the present invention, mayadditionally be ascertained by way of prediction from the properties oftwo component alloys, the alloys AgGaSe₂ and AgGaIn₂. The designSellmeier equation values are conveniently obtained from the Sellmeiervalues for the two component alloys. A commercial source of theAgGa_((1−x))In_(x)Se₂ material is Cleveland Crystal Company.

The AgGa_((1−x))In_(x)Se₂ material is of interest with respect to theSilver Gallium Sulfide Telluride, AgGa(Se_((1−x))Te_(x))₂ and SilverGallium Selenide Telluride, AgGa(Se_((1−x))Te_(x))₂ materials of presentfocus because of its being an early quaternary chalcopyrite materialthat is presently characterized, because of its somewhat predictablecation/anion differences with respect to the Tellurium quaternarymaterials and because it in itself offers advancement of the nonlinearcrystal laser device art over previously used materials.

EXAMPLE 6, AgGa(Se_((1−x))Te_(x))₂

The preceding examples provide “closely related material” disclosurebelieved relevant to the Silver Gallium Selenide Telluride,AgGa(Se_((1−x))Te_(x))₂ and Silver Gallium Sulfide Telluride,AgGa(Se_((1−x))Te_(x))₂ quaternary alloys of principle focus in thepresent document. These examples have, among other things, disclosedcharacteristics of ternary materials usable as parent components forfabricating such quaternary alloys of present document focus.Concurrently these examples also disclose detailed characteristics ofparent ternary materials from which quantitative accurate characteristicpredictions for the fabricated quaternary alloy can be made. The presentexample provides specific additional details regarding one of thefocused-upon quaternary alloys, the alloy Silver Gallium SelenideTelluride, AgGa(Se_((1−x))Te_(x))₂.

Motivation for this approach to the AgGa(Se_((1−x))Te_(x))₂ alloy isprovided by the fact that it is possible to achieve phase matchingoperation in mixed crystals of AgGa(Se_((1−x))Te_(x))₂, for example,when the material component, x, is chosen to provide the desired roomtemperature birefringence. Evidence of this phase matching operation isprovided by the related Silver Gallium Indium Selenide,AgGa_((1−x))In_(x)Se₂ quaternary alloy considered in example 5 above. Inthe focused-upon Tellurium quaternary materials AgGa(Se_((1−x))Te_(x))₂and AgGa(Se_((1−x))Te_(x))₂, however, the adding of Tellurium anions hasseveral benefits over the more traditional example 5 approach of addingIndium cations to a ternary alloy. For example, introducing an anioninto a crystal lattice avoids introducing disorder into thecharacteristic-determining cation sublattice. Also, the added Telluriumions are more efficient than Indium ions in changing the birefringence;therefore substantially smaller x-values are required.

As has been indicated previously herein fabrication of the quaternarySilver Gallium Selenide Telluride, AgGa(Se_((1−x))Te_(x))₂ alloy cancommence with the two ternary alloys AgGaSe₂ and AgGaTe₂ andcharacteristics including Sellmeier equations for these two alloys aredisclosed either directly for the AgGaTe₂ parent or by reference for theAgGaSe₂ parent above. As also noted previously applicants believe theyare the first to characterize the AgGaTe₂ parent material in this detailsince sufficient quantities in necessary quality form were heretoforeunavailable and have only recently become available to applicants.

The indices of refraction for negatively birefringent mixed crystals ofmaterials such as the AgGa(Se_((1−x))Te_(x))₂ of present exampleconsideration for various value of x can be calculated by two differentmodels. Both models have been used separately by writers in theliterature and the methods provide the same result. The Sellmeierexpressions given by equations 13 and 14 for one parent alloy, AgGaTe₂,and the best set of Sellmeier expressions for AgGaSe₂ as disclosed by G.C. Bhar in the journal Applied Optics at volume 15, page 305 (1976) oralternately from Table V above provide the indices of the other parentternary alloy compound in the present instance. In one model, theindices of the mixed crystal AgGa(Se_((1−x))Te_(x))₂ are taken as aweighted average of those of its parents AgGaSe₂ and AgGaTe₂. In theother model, the Sellmeier coefficients for the mixed crystal are takenas a weighted average of those of its parents AgGaSe₂ and AgGaTe₂ usingthe weighting factors of (1−x) and x respectively. FIG. 2 in thedrawings displays the predicted phase matching angles for the negativelybirefringent material AgGa(Se_((1−x))Te_(x))₂ as the Te concentrationvaries from 0-30%.

Equations 15 and 16 provide respectively, quantitative expressions forthe effective NLO coefficient, (χ₃₆ ⁽²⁾)eff, and for the wavelengthconversion efficiency for the AgGa(Se_((1−x))Te_(x))₂ case of negativebirefringence, a Type I SHG/OPO process, and a chalcopyrite using theusual notation and for the standard geometry.

(χ₃₆ ⁽²⁾)_(eff)=(χ₃₆ ⁽²⁾)sinθ  (15)

Conversion efficiency is proportional to: (χ₃₆ ⁽²⁾ sinθ)²/n³  (16)

Non-critical phase matching occurs for a phase matching angle θ=90degrees. For this condition the maximum utilization of the availablenonlinear properties of the AgGa(Se_((1−x))Te_(x))₂ crystal is possibleas sin θ has its maximum value and therefore equation 16 predictsfavorable efficiency.

FIG. 2 in the drawings therefore shows a room temperature nomographfamily of curves relating output wavelength, phase matching angle andTellurium content for a Silver Gallium Selenide Telluride,AgGa(Te_(x)Se_((100−x)))₂ chalcopyrite nonlinear optical material. Withthe experimentally obtained curves of FIG. 2 it is possible to select aset of operating conditions enabling a particular input to outputwavelength relationship for a chalcopyrite crystal of selectedcomposition. Use of the FIG. 2 curves in this manner is perhaps bestexplained with the aid of an example. An example involving the 10.6micron spectral line of the carbon dioxide pump laser depicted at 104 inFIG. 1, and represented by the line 200 in FIG. 2, may be used for thispurpose.

From the labeling associated with the lower scale 202 and the line 200in the FIG. 2 drawing it can be appreciated that the line 200 representsa second harmonic generation (SHG) and wavelength division-by-twooperating condition for a laser device. As a result of these conditionsthe 10.6 micron input wavelength of the pump laser 104 in FIG. 1 isdivided by two and output energy of 5.3 micron wavelength is beingsought-after in the FIG. 2 example. Once these input and outputwavelength relationships have been determined, use the FIG. 2 curvesinvolves ascertaing the percentage of Tellurium content for theoptically nonlinear alloy of the crystal 110 in FIG. 1 using the upperFIG. 2 scale at 208. From this scale it is determined that a crystalhaving about fourteen percent Tellurium content (i.e., eighty sixpercent of maximum Selenium content and fourteen percent of maximumTellurium content of an AgGaSe₂ or an AgGaTe₂ crystal) provides a rangeof possible phase matching angles θ, along the angle scale 204, usablein the laser device under consideration. In making a selection fromthese possible phase matching angles a choice of angles between aboutsixty degrees and ninety degrees is available. It is, of course, usuallydesirable to consider that a large phase matching angle corresponds withhighest conversion efficiency in selecting an operating point in thisrange. The relationship between phase matching angle and conversionefficiency is predicted by several of the equations appearing aboveincluding equations 4, 5, 8, 9, 15 and 16.

From a slightly different perspective the FIG. 2 curves demonstrate thatwith addition of the element Tellurium to the ternary alloy AgGaSe₂ itis possible to operate within a range of wavelengths along the scale 202that are useful in infrared optical parametric oscillation and secondharmonic generation devices. For example, so long as an outputwavelength below 6.3 microns is needed and pure AgGaSe₂ is used in thenonlinear crystal, laser device operation is limited to locations alongthe zero percent Tellurium curve 212 and the only accessible phasematching angles along scale 204 are angles well below the highefficiency, near ninety degree angles. If operation at an outputwavelength of four microns is desired the FIG. 2 curves indicate thatsome Tellurium content is needed if phase matching angles better thanthe forty to forty five degree range are to be achieved.

Clearly operation of a wavelength changing device nonlinear crystal inthe region 210 of the FIG. 2 curves is desirable and the possibility ofsuch operation with its favorable energy conversion efficiency andwithin a desirable four microns to six microns range of outputwavelengths is significantly enhanced by the addition of Tellurium tothe ternary alloy AgGaSe₂. The FIG. 2 nomograph may of course be used toselect the Tellurium content of a Silver Gallium Sulfide Telluridecrystal operative at other phase matching angles than those of theregion 210 if other properties of the crystal or laser device physicallimitations for examples make the ninety degree phase matching angleundesirable.

Curve families of the FIG. 2 type may of course be developed foroperating temperatures other than the room temperature represented inFIG. 2. Similarly curves of the FIG. 2 type may be developed for otherchalcopyrite alloys including the Silver Gallium Sulfide Telluride,AgGa(S_((1−x))Te_(x))₂ alloy of focus in the present invention.

Room temperature wavelength versus refraction index curves for theAgGaTe₂ material, are shown in FIG. 11a of the drawings herein. Thesecurves relate somewhat to the FIG. 2 AgGa(S(_(1−x))Te_(x))₂ curves in abefore and after sense. The disadvantage of the AgGaTe₂ material forpresent purposes that is apparent in FIG. 11a curves, because secondharmonic generation operation is not possible at the wavelength of the10.6 micron carbon dioxide laser line, is corrected through addition ofa near optimum amount of Tellurium to form the present examplequaternary alloy Silver Gallium Selenide Telluride represented in FIG.11b. This formation of the quaternary material has also changed theapplicable index of refraction and the spacing between the illustratedordinary and extraordinary refraction index curves. The illustratedlower value refraction index, as indicated in FIG. 11a, is a valuesupporting second harmonic generation use with the 10.6 micron line.

From the equations 4, 5, 8, 9, 15 and 16 it is also possible toappreciate the benefits of Tellurium addition to a AgGaSe₂ alloy interms of the achieved conversion efficiency. The relative conversionefficiency for AgGa(Se_((1−x))Te_(x))₂ normalized to that of AgGaSe₂ for18% Te is found to be 1.84. As the band gap for this material isestimated to be 1.74 electron volt as compared to a value of 1.83electron volt for AgGaSe₂ and as χ⁽¹²⁾ is known to increase as 1/E_(g)^(4.2) (see A. G. Jackson, M. C. Ohmer, S. R. Leclair, Infrared Physics& Technology, 38, 233 (1997)) its χ⁽²⁾ is estimated to be 24% larger or82 pm/V. As a result, the conversion efficiency for this composition ispredicted to exceed that of the AgGaSe₂ by a factor of 2.3.

The preceding example may be better understood by continuing the line ofthought suggested in the AgGaSe₂ example disclosed above and consideringthat the phase matching angle for AgGaSe₂ is 54.4 degrees for a 10.6micrometers pump and its χ₃₆ ⁽²⁾ is 66 pm/V; these values are reportedin the Springer-Verlag Handbook of Nonlinear Optical Crystals. Thisangle of 54.4 degrees is, however, considerably smaller than the idealphase matching angle of 90 degrees. The value of the angle θ for a 19percent Tellurium addition to this AgGaSe₂ is, however, 86.2 degrees.The figure of merit for conversion efficiency for the 19 percentTellurium composition is found to be a factor of 1.5 greater than thatof AgGaSe₂, considering only the larger angle θ; its χ₃₆ ⁽²⁾ is alsolarger as it has a smaller band gap. The bandgap for an x value of 19percent is estimated to be 1.74 electron volts as compared to a value of1.83 electron volts for AgGaSe₂. As χ₃₆ ⁽²⁾ increases approximately as1/Eg^(4.2), its χ₃₆ ⁽²⁾ is estimated to be 82 pm/V. As a result, theconversion efficiency for this composition is predicted to exceed thatof the AgGaSe₂ by a factor of 2.3 for a 10.6 micrometers second harmonicgeneration. This estimate neglects the additional improvement resultingfrom a factor of ten reduction in the walk-off angle.

Therefore although AgGaTe₂ will not phase match at room temperaturemixed crystals of the form AgGa(Se_((1−x))Te_(x))₂ for Te additions inthe 10-20% range can exceed the conversion efficiency of AgGaSe₂significantly. In addition such a tuned quaternary alloy enablesnoncritical phase match operation of the laser device including use of aphase match angle supporting optimum use of the material's nonlinearproperties, maximized useful length of the available crystal, roomtemperature wavelength changing operation, significantly increasedsecond order nonlinear susceptibility, a factor of ten reduction in thewalk-off angle and photon energy conversion efficiencies several timesthose usually achieved. The Tellurium alloy component also accomplishesshifting of the semiconductor material energy absorption characteristicto avoid a preferred laser pump wavelength energy absorption peak andassists in circumvention of the thermal lensing phenomenon in thecrystal. The accomplished laser device provides infrared energy outputwhile operating in for example either the second harmonic generation orthe optical parametric oscillation configurations. Relatively smalladditions of Te to the present state of the art AgGaSe₂ material improveconversion efficiencies significantly.

The addition of Tellurium to nonlinear optical materials is believedtherefore to provide a significant advance in the nonlinear optical andlaser device arts.

While the apparatus and method herein described constitute a preferredembodiment of the invention, it is to be understood that the inventionis not limited to this precise form of apparatus or method and thatchanges may be made therein without departing from the scope of theinvention which is defined in the appended claims.

What is claimed is:
 1. The efficient stable method for generatingselected wavelength coherent infrared radiant energy, said methodcomprising the steps of: choosing a wavelength converting, reflectivemirror cavity-utilizing, nonlinear optics mode of solid state laserdevice operation for realizing said infrared radiant energy generation;embodying said solid state laser device using a Telluride quaternaryalloy single crystal optically nonlinear semiconductor material in saidwavelength converting-operation; tuning birefringence and absorptioncharacteristics of said single crystal Telluride quaternary opticallynonlinear material to selected values thereof, said tuning stepincluding selecting Tellurium content of said quaternary alloy singlecrystal; withdrawing from an ingot of said single crystal Telluridequaternary material a portion characterized by a selected angle θbetween incident faces and an optical axis, and usable as a wavelengthconverting laser device having input and output optical ports; selectingan input optical energy pump wavelength for said wavelength convertinglaser device, a wavelength corresponding with both phase matching anglenear ninety degrees, operation within said wavelength converting laserdevice and with sought-after infrared laser device output wavelength;said Tellurium content, said optical energy pump wavelength, said phasematching angle and said laser device output wavelength beinginterrelated ascertainable characteristics of said single crystalTelluride quaternary alloy material; exciting said Telluride quaternaryalloy material wavelength converting laser device with opticalpump-sourced optical input energy disposed orthogonally with respect toan input port face of said wavelength converting laser device;maintaining said wavelength converting laser device in a thermallystable operating condition during infrared radiant energy generationuse.
 2. The efficient stable method for generating selected wavelengthcoherent infrared radiant energy of claim 1 wherein said wavelengthconverting, reflective mirror cavity-inclusive, nonlinear optics mode ofsemiconductor laser device operation includes one of second harmonicgeneration, optical parametric oscillation and difference frequencygenerator operating modes.
 3. The efficient stable method for generatingselected wavelength coherent infrared radiant energy of claim 1 whereinsaid wavelength converting, reflective mirror cavity-inclusive,nonlinear optics mode of semiconductor laser device operation includespropagating ordinary ray and extraordinary ray components of said inputport optical energy at identical velocities and substantially zerowalkoff angle within said Telluride quaternary alloy material.
 4. Theefficient stable method for generating selected wavelength coherentinfrared radiant energy of claim 1 wherein said wavelength converting,reflective mirror cavity-inclusive, nonlinear optics mode ofsemiconductor laser device operation includes enhancing energytransferring between Telluride quaternary alloy material input port andoutput port waves by enabling differing polarizations between said inputport waves and said output port waves.
 5. The efficient stable methodfor generating selected wavelength coherent infrared radiant energy ofclaim 1 wherein said wavelength converting, reflective mirrorcavity-inclusive, nonlinear optics mode of semiconductor laser deviceoperation includes noncritical phase matched energy transfer betweendiffering wavelength input and output beams of said Telluride quaternaryalloy material.
 6. The efficient stable method for generating selectedwavelength coherent infrared radiant energy of claim 1 wherein saidquaternary single crystal nonlinear optical material is comprised of oneof materials Silver Gallium Selenide Telluride, AgGa(Se_((1−x))Te_(x))₂and Silver Gallium Sulfide Telluride, AgGa(S_((1−x))Te_(x))₂.
 7. Theefficient stable method for generating selected wavelength coherentinfrared radiant energy of claim 1 wherein said optical device isincorporated in a heat seeldng missile decoy device.
 8. The efficientstable method for generating selected wavelength coherent infraredradiant energy of claim 1 wherein said single crystal Telluridequaternary alloy material includes Tellurium-disordered anion crystalsublattices and normal unmodified cation crystal sublattices.
 9. Theefficient stable method for generating selected wavelength coherentinfrared radiant energy of claim 1 wherein said step of withdrawing froman ingot of said single crystal Telluride quaternary material a portioncharacterized by a selected angle □ between incident faces and anoptical axis includes selecting a crystal cut angle for said Telluridequaternary alloy material.
 10. Laser apparatus comprising thecombination of: a source of coherent radiation pump energy of firstoutput wavelength characteristic; a nonlinear optics wavelength changingsemiconductor crystal element disposed intermediate said source ofcoherent radiation pump energy and a infrared wavelength optical outputport of said laser apparatus; said nonlinear optics wavelength changingelement including a Tellurium-comprised single crystal quaternary alloychalcopyrite semiconductor material having a crystal structurecomprising: a crystalline cubic lattice (304) located at an intersection(306) of 100, 001 and 010 coordinate axes (302, 303, 305) said lattice(304) having lattice initial plane faces (308, 310, 312) received inplanes defined by each 100-001, 100-010 and 010-001 axis pairs, havingsublattice-defming lattice mid planes (314, 316, 318) distal to andparallel with lattice initial planes (308, 310, 312) respectively andhaving exterior face planes (320, 322, 324) distal to and parallel witheach lattice initial plane (308, 310, 312) and each lattice mid plane(314, 316, 318) when viewed along any of three paths (326, 328, 330)parallel to a 100, 001 and 010 axes, paths orthogonal to 001-010,100-010, and 100-001 planes respectively; Gallium atoms (332) located ineach sublattice corner of a lattice mid plane lying along said 100 axis(302) parallel with said 001-010 plane; plus Gallium atoms (334) locatedat each sublattice center in the lattice initial and exterior planes(312, 324) lying along said 100 axis (302) commencing at said 001-010plane; plus Gallium atoms (336) located at sublattice centers of saidinitial, said mid and said exterior planes (310, 316, 322) within afirst half (337) of said cube lattice (304), a half located parallel toand adjacent said 001-010 plane, along said 100 axis (302); plus Silveratoms (338) located at each sublattice corner in lattice initial andexterior plane faces (320, 324) disposed along said 100 axis (302)parallel to said 001-010 plane; plus Silver atoms (340) located atsublattice centers of said initial, said mid and said exterior planes(310, 316, 322) along said 001 axis within a second half (342) of saidcube lattice, a half located parallel to and distal of said 001-010plane along said 100 axis (302); plus one of differentialnumber-quantity, similarly located, Sulfur and Tellurium atoms (343) anddifferential number-quantity, similarly located, Selenium and Telluriumatoms (343) said differential number-quantity atoms (343) being receivedin planes intermediate said lattice initial plane, said lattice midplane and said lattice external plane in mediate planes lying along eachof said 100, 001 and 010 axes, said differential number-quantity atoms(343) being disposed in ordered array in random fill anion latticepatterns paralleling each of said 100-010, 100-001, and 010-001 planes.11. The laser apparatus of claim 10 wherein said crystalline cubiclattice is comprised of a plurality of cubic sublattices, twosublattices in number along each of said 100, 001 and 010 axes.
 12. Thelaser apparatus of claim 10 wherein said differential number-quantity,similarly located, Sulfur and Tellurium atoms and differentialnumber-quantity, similarly located, Selenium and Tellurium atoms aredefmed by the chemical formulations of S_((1−x))Te_(x) andSe_((1−x))Te_(x) wherein x is a mole fraction of Tellurium in saidmaterials.
 13. The laser apparatus of claim 10 wherein said Telluriumatoms are disposed in a cation lattice of said quaternary alloy. 14.Laser device military defensive apparatus comprising the combination of:a source of coherent radiation pump energy of first output wavelengthcharacteristic; a nonlinear optics wavelength changing solid statesemiconductor element disposed intermediate said source of coherentradiation pump energy and an infrared wavelength optical output port ofsaid military defensive apparatus; said nonlinear optics wavelengthchanging element including a Tellurium-comprised single crystalquaternary alloy chalcopyrite semiconductor material having a crystalstructure comprised of: a crystalline cubic lattice (304) located at acoordinate axis intersection (306) of first second and third axes (302,303, 305), said lattice (304) having lattice initial plane faces (308,310, 312) received in first, second and third planes defmed by axispairs, having sublattice-defining lattice mid planes (314, 316, 318)distal to and parallel with said lattice initial planes (308, 310, 312)and having exterior face planes (320, 322, 324) distal to and parallelwith each said lattice initial plane (308, 310, 312) and each saidlattice mid plane (314, 316, 318) when viewed along any of three paths(330, 328, 326) parallel to an axis, paths orthogonal to said firstplane initial face, said second plane initial face and said third planeinitial face; Gallium atoms (332) located in each sublattice corner of alattice mid plane lying along said first axis (302) parallel with saidthird plane; plus Gallium atoms (334) located at each sublattice centerin the lattice initial and exterior planes (312, 324) lying along saidfirst axis (302) commencing at said third plane; plus Gallium atoms(336) located at sublattice centers of said initial, said mid and saidexterior planes (310, 316, 322) within a first half (337) of said cubelattice (304), a half located parallel to and adjacent said third plane,along said first axis (302); plus Silver atoms (338) located at eachsublattice corner in lattice initial and exterior plane faces (320, 324)disposed along said first axis (302) parallel to said third plane; plusSilver atoms (340) located at sublattice centers of said initial, saidmid and said exterior planes (310, 316, 322) along said second axiswithin a second half (342) of said cube lattice, a half located parallelto and distal of said third plane along said first axis (302); plus oneof differential number-quantity, similarly located, Sulfur and Telluriumatoms (343) and differential number-quantity, similarly located,Selenium and Tellurium atoms (343) said differential number-quantityatoms (343) being received in planes intermediate said lattice initialplane, said lattice mid plane and said lattice external plane in mediateplanes lying along each of said first, second and third axes, saiddifferential number-quantity atoms (343) being disposed in ordered arrayin random fill anion lattice patterns paralleling each of said second,first and third planes.
 15. The laser device military defensiveapparatus of claim 14 wherein said Tellurium-inclusive single crystalquaternary alloy chalcopyrite semiconductor material is comprised of oneof the materials of Silver Gallium Selenide Telluride,AgGa(Se_((1−x))Te_(x))₂ and Silver Gallium Sulfide Telluride,AgGa(S_((1−x))Te_(x))₂.
 16. The laser device military defensiveapparatus of claim 14 wherein said apparatus comprises one of a chemicalcontaminant and a biological contaminant detection apparatus.
 17. Thelaser device military defensive apparatus of claim 14 wherein saidapparatus comprises an aircraft heat seeking missile defensive decoyapparatus.
 18. The laser device military defensive apparatus of claim 14wherein said wavelength changing solid state semiconductor element ischaracterized by a second harmonic generation function and wherein saidsource of coherent radiation pump energy includes a source of radiantenergy of longer wavelength than said second infrared wavelength opticaloutput port energy.
 19. The laser device military defensive apparatus ofclaim 14 wherein said wavelength changing solid state semiconductorelement is characterized by a optical parametric oscillation functionand wherein source of coherent radiation pump energy includes a sourceof radiant energy of shorter wavelength than said second infraredwavelength optical output port energy.
 20. The laser device militarydefensive apparatus of claim 14 wherein said nonlinear optics wavelengthchanging solid state semiconductor element is characterized by operationin a noncritical phase matching and input to output beam walkoff freeoperation.
 21. The laser device military defensive apparatus of claim 14further including temperature maintenance apparatus operativelyconnected intermediate said nonlinear optics wavelength changing solidstate semiconductor material and external energy apparatus.
 22. Solidstate wavelength changing laser apparatus comprising the combination of:a chalcopyrite Tellurium anion-inclusive, quaternary alloy, opticallynonlinear single crystal of semiconductor material having selectivelydisposed optical input and output ports; stimulated emission opticalenergizing apparatus of first output wavelength disposed in photoncoupled energy communication relationship with said semiconductormaterial crystal input port; said stimulated emission optical energizingapparatus having energy output from said output port of wavelengthdifferent from that of said semiconductor material crystal input port;crystal temperature maintenance apparatus operatively connectedintermediate said crystal material and external energy apparatus. 23.The method of providing solid state wavelength changing apparatus ofinfrared output wavelength capability, desirable photon energyconversion efficiency and energy conversion beam walkoff immunity, saidmethod comprising the steps of: fabricating a single crystal quaternarychalcopyrite alloy crystal material inclusive of periodic table elementsSilver and Gallium and one of elements Selenium and Sulfur as awavelength-converting nonlinear optical element of said laser apparatus;said single crystal quaternary chalcopyrite alloy crystal materialincluding crystal faces meeting in an angle selected for noncriticalphase matched input and output beam propagation in said crystal;including anion sublattice atoms of element Tellurium in said selectedsingle crystal chalcopyrite alloy material; selecting adjacent input andoutput port faces of said crystal meeting in an apex angle supportingnoncritical phase match energy exchange between input and output beamsof said crystal; adjusting birefringence optical characteristics withinsaid single crystal chalcopyrite material for a condition of noncritical phase matching between laser pump beam and laser signal beamtravel therein; maintaining said single crystal chalcopyrite materialwithin a temperature range enabling continuation of said adjustedbirefringence optical characteristics and said non critical phasematching during operation of said laser apparatus.